"Who's to say what blatant nonsense is though a few hundred years ago it was blatantly obvious the world was flat."
It was blatantly obvious to any coastal civilization whose citizens had watched boats sail over the horizon that the world was round. The ancient Greeks knew it, as I'm sure did others.
"I don't see how it is harmful for someone to go see this guy even though it is complete nonsense."
The guy claims to treat major diseases like cancer. It's harmful for someone to go see this guy because he's a fraud, he takes their money, is incapable of offering actual help, and the patient dies.