If one has only the quotes from Bp Williamson on this short video, I'm not certain that H.E. makes an airtight argument for the rejection of sedevacantism. I know several sedes who would be highly offended at the notion that their views on the post-conciliar papacy are basically "liberal." The idea that we can reject almost everything that conciliar popes and bishops do and say, while, at the same time, recognizing their God-given authority and office, is highly problematic for many traditional Catholics. I think that Williamson has sympathized in the past with the contradictions posed by the present situation, and has cut some slack to the sincere sedevacantist. I don't think that he rejects or their position out of hand. He is a bit more tolerant than that. But hey, if I misrepresent his sentiments, then please offer correction.
In the end, a genuine pope, in concert with genuine bishops, must make the consecration of Russia to the Immaculate Heart of Mary. Meanwhile, the record seems to be replete with statements from no less than ABL himself that the Vatican is in all but total apostasy, and that the highest posts in the church are occupied by virtual "anti-Christs."