If you know, a manifest heretic is being named the Canon - you cannot go. If he is a "sede" he KNOWS. One cannot ignore a certain conscience, and if you know the one so-named is not pope - it is a lie and a profanation.
If you read the quote, this was about Protestant baptisms and how they would be materially wrong about where the true Church is but they FORMALLY believe in a true Church. Even though we know they are materially wrong, they are formally right which is why we would be allowed to participate and unite ourselves with one of them if they had to perform an emergency Baptism on a Catholic. If it were intrinsically wrong, the Church wouldn't allow such a thing, which means it is NOT intrinsically wrong for there to be material error, but be formally correct. It's the same with a sede going to a Mass with the wrong pope accidentally mentioned - they are materially wrong, but formally correct in wanting to abide by the rule of the Church to name a true pope, and this is not instrinsically evil.