CathInfo.com

To be fair, Matthew, +De Lauriers wrote that letter in 1979. Whether what is contained in it is true or not, Archbishop Lefebvre certainly had the opportunity to defend himself

Here's a start, with the May 1980 issue of Itineraire attached (in French, sorry), with Jean Madiran addressing Fr de Lauriers' letter.

Here is a Deepl translation of the Archbishop's letter from this issue:

Feast of Pentecost
3^rd June 1979

My dear friends,

You have asked me to reply to Father Guérard des Lauriers' letter in the magazine “Einsicht”.
It reminds me so much of the scene suffered by Our Lord at the hands of the soldiery that I prefer to remain silent like the Divine Master and to pray for those who persecute us.
However, I can affirm that I have never celebrated the new Mass according to the rite introduced in November 1969.
I have always considered it dangerous and poisoned by a false ecuмenism.
I think that this statement and the liturgical life of our seminaries and priories should be enough to reassure those who have any doubts.
Let us not fear insults and slander, but only wrongdoing.
Yours faithfully in Christo et Maria

[End of full reproduction of Archbishop Lefebvre's letter, published in French and German
August 1979, Mitteilungsblatt der Priesterbruderschaft St. Pius X für den deutschen Deutschland.
Pius X für den deutschen Sprachraum, published at: Priorat St. Pius X, 8000 München
80, Hohenaschauerstr. 98].

Here is a 2023 article from the SSPX website addressing the accusation. As is common with the new SSPX it is anonymous:

https://sspx.org/en/news/archbishop-lefebvre-never-celebrated-new-mass-28679

Archbishop Lefebvre Never Celebrated the New Mass
January 30, 2023
Source: District of the USA
       
Sometimes one hears the claim being made that Archbishop Marcel Lefebvre, founder of the Society of Saint Pius X (SSPX), celebrated the New Mass for a time before devoting himself exclusively to the traditional Mass. The purpose of this article is to show that this claim is false.

The Society’s Liturgical Disposition
As recounted in Bishop Bernard Tissier de Mallerais’s authoritative Marcel Lefebvre: The Biography (Angelus Press 2004), “Since Archbishop Lefebvre was opposed to the New Mass, he would not have it in the seminary” (p. 416). When the first crop of seminarians who would form the foundation of the Society arrived in Fribourg, Switzerland in 1969, it was decided quickly that the Old Mass, that is the traditional Latin Mass (TLM), would be celebrated by the nascent priestly fraternity rather than the New Mass of Pope Paul VI. After the New Mass was introduced into the Diocese of Fribourg on the eve of Advent 1969, the Archbishop continued to celebrate the TLM, though some of the seminarians assisted at the New Mass in Latin at a nearby Bernardine monastery on Sundays when the Archbishop was called away on other matters.

The Archbishop’s decision to allow his seminarians to assist at the Novus Ordo Missae was nothing more than an expression of his prudence. Not wanting to cause scandal to the faithful by appearing to disobey the local bishop, Archbishop Lefebvre took a cautious approach at first, informing an inquiring priest that the Roman Canon should be retained while the words of consecration are said in a low voice. As time went on, the Archbishop’s concerns over the New Mass grew as he observed its disastrous fruits, consulted with episcopal colleagues, and reflected more on its doctrinal distortions.

While much could be written on the liturgical praxis of the SSPX and other traditionalist Catholics during the years following the New Mass’s introduction, many priests simply relied upon the missals, breviaries, and other liturgical texts that were available to them. In 1965, for instance, several alterations were made to the 1962 Missale Romanum, such as the abbreviation of Psalm 42 during the so-called “Prayers at the Foot of the Altar”; the use of the vernacular for the epistle and gospel readings; and the suppression of the Last Gospel. The Archbishop accepted these slight modifications to the traditional Mass at first before finally settling on the 1962 books promulgated by Pope John XXIII as the SSPX’s normative liturgy in 1974.

False Accusations
At no point did the Archbishop himself ever say the New Mass. It appears that the calumny saying that he did has its origins in an unsubstantiated letter written by Fr. Guérard des Lauriers, OP, on April 12, 1979. Fr. des Lauriers was a French theologian and professor who had formerly taught at the SSPX’s seminary in Écône, Switzerland until his embrace of sedevacantism in 1977. Not only did the Archbishop deny this charge, but it was refuted in detail by Jean Madiran in the May 1980 issue of the journal he founded, Itinéraires. Madiran called Lauriers out for his outrageous language against the Archbishop, namely, accusing him of being a traitor and another Pontius Pilate. However, he noted the claim that the Archbishop celebrated the New Mass from April, 1969, to December, 1971, as being the worst accusation of all.

Madiran pointed out to Fr. Lauriers that the New Mass was not even permitted before November 1969, and that it was absurd to think that the Archbishop was such an enthusiast for the New Mass that he would have started celebrating it before everyone else.
In the end, when the Archbishop wrote to Fr. Lauriers to deny the charge, the latter wrote back to say that he was happy this was the case, but that the Archbishop had made certain gestures while celebrating the traditional Mass that made him think he was celebrating the New Mass. It was clear at this point that Fr. Lauriers’s original letter was a case of bad polemics.

Consequences of the Archbishop’s Decision
The Archbishop’s decision that he and his priests would not celebrate the New Mass had immense consequences for the Church. It is clear, in hindsight, that the Roman authorities would have been happy for the traditional Mass to disappear completely from the life of the Church, despite the fact that its origin is too ancient to be pinpointed precisely and that it had been the cultural bedrock of Western civilization for over a millennium. If the Archbishop had not refused the New Mass and formed generations of priests in the old Mass, we may ask whether the TLM would even be available in 2023.

Furthermore, it was surely the Archbishop’s perseverance in the formation of traditional priests for the SSPX from 1970-88 that caused the Roman authorities to decide to make some place, even if just a little one, for the “Mass of All Time” in the life of the Church. This was the primary reason why the 1962 Missal, the “Mass of Écône,” received specific Roman approval in the indults issued by Pope John Paul II in 1984 and 1988. It was the reason why Pope John Paul II, and Pope Benedict XVI after him, allowed for the canonical erection of the so-called Ecclesia Dei communities, which were established celebrating the TLM exclusively.

It is clear that the Conciliar churchmen would have been quite content for the TLM to vanish and that they had no will to grant concessions for it. Only pressure applied from the outside made them change their minds. This pressure came primarily from one source: the Archbishop’s fidelity to the old Mass, as well as the fidelity of his sons. It was those same sons who asked Pope Benedict XVI to provide universal authorization for the TLM, a request which he famously granted with his 2007 motu proprio, Summorum Pontificuм.

The Archbishop’s Liturgical Legacy
Traditional Catholics the world over continue to feed on the spiritual harvest planted by Archbishop Marcel Lefebvre over 50 years ago. Whether or not they assist at Masses offered by the priests of the SSPX, these faithful are able to receive the sacraments according to the 1962 texts because of the Archbishop’s choice to remain faithful to the never-abrogated Mass of All Time, before the disastrous intrusion of the Novus Ordo Missae into the Roman Catholic Church’s liturgical life. Had the Archbishop capitulated as his calumniators contend, there is no telling what state the traditional liturgy would be in today.

May this history of the brave Archbishop’s refusal of the novel liturgy of the New Mass, which has been so damaging to the eternal salvation of souls in the past 54 years, be a source of encouragement to all those who are tempted to sacrifice the interests of the Catholic Faith in difficult circuмstances.

sspx.org - 01/30/2023

I think some Trads have their feelings hurt because they didn't end up in such a great position in "Tradition 2.0" after the fall of the SSPX -- they didn't get very lucky with regards to where all the Resistance priests ended up.

So they go and align themselves with enemies of the SSPX AND the Resistance. And that is NOT good.

I’ll never understand the cινιℓ ωαr within Tradition.  Sede vs R&R.  Sure, there are disagreements and theological disputes between camps.  But 90% of Trads have been to both “sides” chapels due to traveling or necessity.  

And yet, an R&R Trad will walk out of a Sede chapel where he just happily attended Mass and then go online and post how the Sedes are quasi-heretics.  And the Sedes do the same thing.  

It's the most immature and insane thing I’ve ever experienced.  And…as things get worse in new Rome, the NORMAL effect would be that Trads band together.  But, no, the divide widens (even among Sede vs Resistance).  

So for Trads, “the enemy (other Trads) of my enemy (new-Rome) is NOT my friend.”  Makes no sense.  We don’t deserve a Catholic pope.  Most of us deserve a Muslim occupation.