Sedes don't judge popes. They judge that a particular man cannot be, or could not have been, a true pope. Entirely two different things.
I don't even think that the judging the person suffices. I think that sedevacantists simply judge that the Conciliar Church is not the Catholic Church by any stretch, and since it was created by these putative popes, they cannot have been popes, or at least, freely functioning as popes.
I do think the "personal manifest heresy" angle is a total dead end.
There was very little there for the Church to have judged Roncalli a pertinacious manifest heretic. Despite his file having labeled him as "suspect", he was still permitted to hold high office in the Church, and even made a Cardinal.
Then when it comes to Montini ... I have not yet seen a shred of evidence from before he was "(s)elected" demonstrating that he was a manifest heretic. If such can be found at this time, it's so obscure as to not have qualified as being sufficiently manifest to render him a non-member of the Church. But thus far no one has ever even presented anything. You can't just depose him after he starts teaching.
At the end of the day, it's the Siri Theory that explains everything ... with nothing else coming close.