Yes, he is guilty. But he's guilty of defamation, not incitement. It is an important distinction because "I thought that was true" is a defense in a defamation case, but not in incitement- you can't say "the pope is catholic, go kill him now", regardless of whether he is actually catholic.
"He didn't present a defense therefore it has been determined for a fact that he is guilty" is not especially sound. You'd have to concede the existence of witches on the absurd end, and that everyone who makes a plea deal is guilty on the more rational end. He's guilty because he publicly made harmful defamatory statements that he privately did not believe, both of which are made clear by evidence.
"He didn't present a defense therefore it has been determined for a fact that he is guilty" is not especially sound. You'd have to concede the existence of witches on the absurd end, and that everyone who makes a plea deal is guilty on the more rational end. He's guilty because he publicly made harmful defamatory statements that he privately did not believe, both of which are made clear by evidence.