If a man walks up to me, puts the barrel of a gun on my forehead and says, 'hand over your wallet', but then smiles and says he has my 'best interests' in mind, I simply believe him? What if he does this for years in public to everyone, on every street corner in town?
Call a spade a spade. That's precisely why there was a Holy Office and that's what the Holy Office had been doing for years - calling warped theologians warped in their non Catholic Modernism.
The mugger would have to have some kind of explanation. He might present an explanation that would lead you to think, "Wow. This crazy guy really thinks he's helping me. Poor dude, he's doing wrong and needs to be put in the madhouse."
Benedict was asked to explain why he was saying the Church used to say one thing and now says another. His explanation was fallacious, but you can see why someone with his formation would believe his explanation. That makes him a sincere crackpot, not a formal heretic.
Of course, it's possible he was extorting our good will, and knew perfectly well his explanation was bogus. However we have very good reason to believe he was just a sincere crackpot.