In my real world experience it's easy to fix. Most spam is generic. Publish a blog post asking applicants to include a specific keyword in the subject line. That sorts out 80% of the spam. (probably all of it)

Asking for a cover letter in docx format, requesting info on the format of the book group, and what other authors they have discussed recently, sorts out another 99%.

Filter both these out and you are left with a small number of applicants. If someone is a tailoring an AI to defeat this, then author has a very high value event on his hands that he should hire someone to help organise.

If applicants are not willing to do this, then they clearly are not offering a high-value opportunity in the first place. His excuse obviously fools most people, hence your reply, but it's very unlikely to be the big picture in my view. He just doesn't want to do the book group. Not enough to set up some simple filters anyway.