It seems that you are incorrect, as I find +Williamson using the same principle way back in 1996 (and basing it on +Lefebvre himself):
”Q: But does not Michael Davies say that attending the Novus Ordo Mass fulfils one’s Sunday duty? And that Archbishop Lefebvre said the same thing?
A: When Michael Davies says it, it is because he claims that the officially promulgated Novus Ordo Mass cannot be intrinsically evil, otherwise the Catholic Church would be defectible.
When Archbishop Lefebvre said it, he meant that the Novus Ordo Mass is objectively and intrinsically evil, but Catholics unaware of, or disbelieving in, that evil, because of the rite’s official promulgation, may subjectively fulfil their Sunday duty by attending the new Mass. The third Commandment says, thou shalt keep the Sabbath holy, not, thou shalt attend a semi-Protestant Mass.”
(Bishop Williamson, Letters of the Rector of St. Thomas Aquinas Seminary, December 1, 1996)”
1) Can you explain how +Williamson’s Mahopac advice to the ignorant conciliar woman was any different than the application of Lefebvre’s principle above?
2) Can you explain how Lefebvre can say one can fulfill their Sunday obligation by committing -as you claim- an intrinsically evil moral act?
Clearly, the Williamson of 2015-2023 is the same as that of 1996 (and the same as Lefebvre).
PS: I’ll let you flounder with #2 for a couple hours, before explaining and resolving your confusion, since this seems to be your foundational error.
Since #1 is self-evident, I'll address #2:
Obviously, the precept of the Church is predicated upon the Commandment "Thou shalt keep holy the Lord's day."
The Church teaches this commandment is minimally satisfied by attending Mass, and consequently, if one has attended Mass, one has sufficiently kept holy the Lord's day (notwithstanding extraneous sins outside the scope of this discussion).
Consequently, were it true that ATTENDING the new Mass was an intrinsically evil/immoral act, it could in nowise be excused or permitted, regardless of circuмstances, and consequently, one could not fulfill the precept by attending the new Mass, for any reason.
The sedevacantist says this simply shows the confusion, error, or inconsistency of Lefebvre.
The Hewkonian/Pfeifferite looks past Lefebvre, but attributes the same to Williamson.
I say otherwise (to both):
The word "intrinsic" is inherently ambiguous, as it exists in multiple senses (both of which touch upon different aspects of this question):
1) There are intrnisically evil
human acts (e.g., abortion; sodomy; etc.).
2) There are intrinsically evil
things in the scholastic/philosophical sense of
evil as a deprivation of a good integral to its nature (e.g., a one-legged man; a two-legged chair; a rite of Mass without a proper offertory or reference to a acrificial priesthood; etc.).
The error inspiring opposition to +Williamson (and unwittingly to +Lefebvre, per the quote above), is the conflation of the two senses of "intrinsic," and improperly considering the former instead of the latter.
The SSPX, +Lefebvre, and +Williamson have always referred to the new Mass as intrinsically evil in this latter, philosophical sense (i.e., intrinsically evil as a deprivation of a good integral to the nature of a thing).
Were it not so, +Lefebvre could not say that the ignorant (or those in necessity, per his cσncєnтrαтισn cαмρ analogy) can fulfill their Sunday obligation by attending it, since this would be tantamount to him saying that one can keep holy the Lord's day by commiting an intrinsically evil human act (i.e., a sin), which is preposterous.
Going back to Mahopac, we find +Williamson applying the same principle.
The problem seems to be that the simple faithful, not understanding the distinction between species of "intrinsic evil," rallied to defend a principle Lefebvre and Williamson never held (and scandalized upon learning this, as a result of many years of poor formation, choose to rally around their error, rather than correcting it). That it was foreseeable such would happen is why I say Williamson was guilty of a minor imprudence.
But the fact of the matter is that both Lefebvre and Williamson are perfectly correct, and if the Hewkonian/Pfeifferites fanned the flames of ignorance and sectarian partisanship to get (and keep) their movements going, the fault lies with the latter, and not the fformer.