The response of "I'm not debating this with you" is sane.
Not really. Or, not reasonable or responsible, I would say.
Not wanting to have a public debate about it sounds fine. Hearing the CEO out and then saying "I'm sorry but it sounds like we're just not on the same page on this and talking more is not productive" sounds fine.
Deciding to make statements in a public forum that could materially impact someone's livelihood, and not being at all open on any level to someone who wants to tell you hey I think some of these statements are just factually wrong and this person you're convinced is a bad person and are publicly saying is a bad person, is not actually a bad person, that seems like middle-school levels of weird and hostile and self-centered.
Imo he should've just let the guy be after he told him to not contact me again
Yeah. In my armchair mode, I feel like at that point you should back off and maybe make a short public response like "Hey I saw this and tried to reach out privately, not to get into a big back-and-forth but here's why I think most of this is a misunderstanding of what we're about. (point a, b, c) Happy to talk more if you change your mind."