You can have an AI help write it while still having done research yourself, they're not mutually exclusive. I'm not claiming whether he actually did that or not.

Like, for me personally, I'm terrible at writing, so I'll gladly have some AI throw together a draft, which I then verify and edit.

exactly. If I am writing a free book with solid information, well, I am not writing it, because I am not writing free books -- I don't have time. But if AI can arrange my data, thoughts, perspective, and articulate it effectively, then I will let AI write the free book. If the book contains new, or previously unarticulated data and insight, then why not? But I see the very thought is hotly contested here.