I can think of 2 reasons. First, the 100 books in a year was entirely possible in the pulp fiction genre even 20 years ago. If you're mass producing crap that people binge read on their daily train commute, you can write it quickly as they don't much care about the quality. Secondly, Amazon being over run with book slop isn't a new phenomena. As soon as print on demand became a thing 15+ years ago, people starting filling Amazon with junk, either poor copies of other people's books or even just books containing nothing but wikipedia entries. The model had so low a bar to entry, that even if they only sold a couple of each title it was worth their time, but it made Amazon awful to use as a customer trying to browse for a book that might be interesting.