'Sedevacantism' as a 'pestiferous heresy'? Fr Ripperger's accusation analysed

Some accusations, especially made by men of prominence, require a response.
‘Pestiferous heresy’?
In a sermon on the Ascension, seemingly published on Ascension Thursday or the following Sunday, Fr Chad Ripperger has described “sedevacantism” as a “pestiferous heresy”. “Sedevacantism” is a name given to the theological position that the post-conciliar claimants have lacked papal authority since at least 1965.
In many cases, it is best to pass over accusations of this kind. In his Introduction to the Devout Life, the Saint and Doctor of the Church St Francis de Sales – himself an authority for the opinion of ipso facto loss of office – advises his readers how to respond to accusations. He says that we should not be “over-nice in regard to the preservation of our good name”, and that “overlooking and despising of an injury or calumny is, generally speaking, by far a more effectual remedy.”1
However, the Saint and Doctor of the Church concludes the relevant chapter as follows:
“I …