But it is my understanding that the FSSP must not speak against Vatican II despite its errors. This was a requirement for its formation. So they really can’t say what needs to be said.
What is going on here is not just about the availability of the Latin Mass. Until others wake up to this, we will continue to witness what is going on in the OP. It seems as if the Latin mass continues to be used as a pawn by the Modernists in the church: We’ll let you have the “extraordinary” mass, but you can not speak the truth about VII.
The errors of Vatican II will be corrected by Vatican III, unless other circumstances make that a moot point. And personally, I think the latter "other circumstances" scenario might be the more likely one. Vatican II was a deliberate strategy to "Protestantize" the Catholic Church. Fifty years later, the institutional Catholic Church as we knew it has made great progress in that regard ... which means it is disappearing just like all the other Protestant congregations.
In several decades, groups like the FSSP and SSPX will be the only Catholic institutions left in many western countries. They won't even need permission from local bishops to open their own apostolates because nobody will care enough about "Catholicism" to oppose them.
Although I had not heard that FSSP must not speak against VII, I think there are those who could, or would, infer that given that FSSP is clearly in line with the Pope and the Roman Church.
Whether that is a formal directive or tenet, I can’t say, but then my rare experience with FSSP is really through the beautiful Traditional Latin Mass. They rather appear (to me) to keep their head down and tend to that mission of celebrating the Mass, and diligently to the other sacraments and the old blessings of candles, people, places and things.
How engaged they are in worldly opinions, or even worldly affairs, I just can’t tell. I believe they would not enjoy the increasing success they’ve known, however, if they were outside the invitation and approval of the several bishops who are not blocking the Summorum Pontificum.