If I'm not mistaken, Bishop Williamson himself admits to doubt as to whether or not Bergoglio is the pope. (He has done this many times, even while Fr. Gruner was alive.) This also, if memory serves correctly, was stated in his most recent interview with Luigi. Bishop Fellay has stated that one day we may say that Bergoglio was not the pope. I don't know a Traditional group that does not doubt the legitimacy of the Conciliar church papal claimants.
People don't understand how critical this is. Normally a pope's legitimacy is known with dogmatic certainty, as a dogmatic fact. So when comments of this nature are made, this is not actually sedeplenism, but something in between, which I have termed sede-doubtism. Sedeplenism in the strict sense requires dogmatic certainty, which means that one could no more speculate that Bergoglio is not pope than one could speculate that there might only be Two Persons in the Holy Trinity.
If you were to ask any Traditional Catholic, do you believe with the certainty of faith that Bergoglio is pope, i.e., are you as certain that Bergoglio is pope as you are that Our Lord is present in the Blessed Sacrament ... you'd get a loud cry in unison of heck no. At best people may have some personal moral certainty. For most Traditional Catholics, Bergoglio falls squarely into the category of
papa dubius ... which of course resolves into
nullus papa. Sede-doubtism reduces, due to the lack of dogmatic certainty, to a practical sedevacantism, the
nullus papa.
Moral certainty does not count. If Bergoglio were to come out tomorrow and solemnly define something, people who don't have certainty of faith regarding Bergoglio's legitimacy could not have the requisite certainty of faith regarding that dogma. That's why theologians held to the maxim of
papa dubius nullus papa.