The error here is the conflating of personal opinions and legitimate discourse with an official position of an organization.

Some official publications where the SSPX and Archbishop Lefebvre expressed their opposition to Sedevacantism include (according to ChatGPT):

1. “Open Letter to Confused Catholics” (1986) by Archbishop Marcel Lefebvre - In this book, Lefebvre criticizes the post-Vatican II reforms but maintains that the pope is still the legitimate pontiff.
2. “The Mystery of the Church and the Pope” (1982) - A collection of texts and conferences by Archbishop Lefebvre, where he discusses the crisis in the Church and the role of the pope.
3. “They Have Uncrowned Him” (1988) by Archbishop Marcel Lefebvre - This book elaborates on his views about the Church’s crisis and reiterates his recognition of the legitimacy of the recent popes.
4. SSPX’s official publications such as “Fideliter” magazine - Various issues of this periodical included articles and interviews where Lefebvre and other SSPX leaders clarified their stance against Sedevacantism.
5. SSPX District Newsletters and official statements - Throughout Lefebvre’s life, the SSPX released numerous statements and newsletters addressing their position on various issues, including their opposition to Sedevacantism.

On whether SSPX priests should discuss the pope question with the faithful and educate them that sedevacantism is a possibility:

“ Well. I told the priests, in Paris, when I gathered them, and then to you, I had already told you, I said: ‘I think that, little by little, we must still enlighten the faithful a little…’
I do not say that it should be done brutally, and throw this to the faithful to frighten them… No. But I still think that it is precisely a matter of faith.
The faithful must not lose the faith. We are responsible for keeping the faith of the faithful, protecting it. They will lose the faith… even our traditionalists.”

That’s a rather striking contrast to the SSPX position since Lefebvre’s death, in which any discussion of the subject has run in the exact opposite direction (ie., dogmatic anti sedevacantism).

Lefebvre (on whether the pope is the pope):

“I do not know, I am not making a decision! But you can ask yourselves the question. I think that any sensible man must ask himself the question. I do not know.”

To me this seems very reasonable and prudent.