The proper \@question is whether other Roman Catholics consider sedevacantists to be part of their same faith. I suppose it could depend upon which papal doctrines and practices they considered heretical or apostate.
In the example I saw earlier, it seems that the OP considered Pope Francis to be heretical for seeming to agree with Martin Luther (founder of the Lutheran Church) that salvation is by faith and not by works. Not having read what the Pope said, I cannot even say that he agreed. It could easily be a “we disagree, but we’ll wait until the afterlife to see which is correct” situation.
I suppose I should start by reading what Martin Luther wrote. Assuming that I am curious enough to do so, of course.