I don't recommend bad writers write technical books. I also don't recommend people who don't read regularly write technical books. Ultimately writing a good technical book requires a lot of the same things as writing any other kind of book: dedication, perseverance, writing skills, and attention to detail (and let's not forget extensive domain knowledge). It's not doable by most people given their time constraints, IMHO.
The author's assumptions about compensation are off though. You're lucky if you sell 2000 copies in most technical markets, and your royalty rate is above 12%. Forget that if you have a coauthor. Your main compensation is indeed the brand building unfortunately (plus a small advance).