Archbishop Lefebvre held the possibility that the popes were not true popes. We know that already. That's very different from what the Sedes think. They (you) are certain that Francis, for example, is not the Pope, isn't that correct? Sedes deal in certainties, in that everything has to be black-and-white. Yet Archbishop Lefebvre's position is not black-and-white.
I have a copy of Archbishop Lefebvre' book, "Open Letter to confused Catholics," published in 1986, two years before the consecrations of 1988, and twelve years after the quote you provided. In chapter XXl of the book, on page 176, +ABL writes:
"Blind obedience is not Catholic; nobody is exempt from responsibility for having obeyed man rather than God if he accepts orders from a higher authority, even the Pope, when these are contrary to the Will of God as it is known with certainty from Tradition. It is true that one cannot envisage such an eventuality when the papal infallibility is engaged; but this happens in only a limited number of cases. It is an error to think that every word uttered by the Pope is infallible.
"Nevertheless, I am not among those who insist or insinuate that Paul Vl was a heretic and therefore, by that very fact, no longer Pope. John Paul I and John Paul ll would not then have been legitimately elected. This is the position of those so-called "sede-vacantists."
"It has to be admitted that Paul Vl has posed a very serious problem for the consciences of the faithful. This pontiff has done more harm to the Church than the French Revolution. There are definite acts of his, such as his signature to Article 7 of the Institutio Generalis of the New Mass, and likewise to the Council's docuмents on Religious Liberty, that are scandalous. But it is not a simple problem to know whether or not a Pope can be a heretic. A good many theologians think he can be as a private teacher but not as a teacher of the Universal Church. We have to consider the degree to which the Pope intended to invoke his infallibility in such cases as I have quoted."
That's the difference between the SSPX and the Sedevacantists. The SSPX is devoted to the Archbishops interpretation and implementation of tradition, the sedevacantists are dedicated to the theological ramifications of consistent thought.
For example, there are actually three distinct elements that Can be treated in the loss of authority-
Excommunication
Deposition
Resignation
Now these three things are not the same. To be excommunicated is to be placed outside the Church by a legal act.
To be deposed means to be stripped of your faculties to exercise jurisdiction and celebrate the sacraments by a legal act.
To resign means to give up juridical authority either tacitly or actually.
Now, strictly speaking even a publicly heretical Pope is not excommunicated, because no competent authority has placed him outside the Church.
Neither is he deposed, because no competent authority had stripped him of his faculties.
Rather, what is recognized here is that through his PUBLIC and NOTORIOUS heresy, he no longer professes the Catholic faith and therefore tacitly he resigns his office. This means he has no juridical authority which he can exercise, he is a corpse in terms of authority.
What remains is for a competent authority to declare this and implement this should he not repent.
The question then becomes can inferiors recognize when superiors have abandoned their office?
Yes, and we have an example of this in the life of St. Hypatius who, after having been made aware of the preaching of Nestorius, refused to commemorate him in the liturgy, because he considered him to no longer be a Catholic bishop.
And his stance in this matter was vindicated by Pope. St. Celestine declared that from the moment that Nestorius had begun to preach heresy, all his juridical acts were invalid. He had no authority from that moment on, from the moment he became a public heretic.
This is all that Sedevacantists say about Francis and the other Popes- from the time they publicly espoused heresy and manifested it notoriously, they abdicated the throne tacitly and lost authority.
For that reason, they can be deposed.