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.

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?

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.