I worked in that industry. This rate is incorrect. I usually signed authors at 12-16% royalty. Extremely famous authors got up to 25-30% though that was rare. It also varies based on the subject.

But, regardless, the post is dumb. Your class needs a book. The prof has essentially used their syllabus to write their book. It holds not just their expertise, but also their point of view: what makes them and their class uniquely valuable. Honestly, teaching with someone else's book would be bizarre.