You shouldn't trust a book recommendation of the recommender doesn't explain why they are recommending it.

If you write your own textbook, you had better be prepared to explain why your book is better than other books on the same subject.

The author did, though you may not find it satisfactory. In particular, people attacked him for recommending his book, with little specific counterargument ("it's terrible" isn't good enough). If they had engaged him in a dialog instead of insulting, maybe we'd have a better justification in the list.

I kinda feel the most ethical thing for an author to do is focus on a recommendation for the best book that isn't theirs (e.g. besides my book X, I'd recommend Y for Z reasons).

Why would they have bothered writing the book if others existed?

Or, if better books existed after they wrote theirs, why would they bother mentioning theirs at all?

You can acknowledge that someone else's book is well-written, accurate, and valuable even if you think yours is better, or has a different focus, or a different pedagogical method, or...

The rules for the list don't say that an author can't recommend his/her own book. The rules do say that a recommender must explain why he/she recommends one book over others, which this author did.