What's the basis for calling him a grifter? I don't know anything about the guy, but grifting usually involves tricking people for money, which includes both receiving money and knowing you're being deceptive. Just based on this story, his behavior is consistent with a guy who is wrong but refuses to admit it, even to himself. That's not a grifter, that's something much more common!
And while I know nothing about this particular guy, it's relevant because calling people names and making fun of them usually just makes them more entrenched in their position. It makes them less likely to admit they were wrong. If the goal is for the guy to say "yep, I've been making a huge mistake" and then try to get him to help spreading the correct information to the people who listen to him, then there needs to be room for him to do that safely, with a minimum of public humiliation.
I just don't like the tone of mocking and condescension. "Interesting" could be a good start for someone changing their mind, if handled right.