".....depending on how one judges the manner in which he was elected."
Bishop Williamson is hinting, whether knowingly or not, at the root of the problem here, and that is that Jorge Bergoglio was "elected" by the cardinals while there was a pope, Joseph Ratzinger, still occupying the Throne of Peter. However, if it can be demonstrated with sufficient evidence that Jorge Bergoglio was a public manifest formal heretic BEFORE his "election", then that would be the a priori cause (reference cuм Ex Apostolatus Officio) of his not being pope.