I was also skeptical of this claim but managed to find someone who explains it:

https://downtownbrown.substack.com/p/five-fallacies-ai-and-d...

It's not that individual companies buy all copies of a given book, but that there's more than one book scanning company, and they aren't sharing the scans with each other. The result: books that were rare but nevertheless easy to find for purchase (thanks to the internet) are now vanishing off of the market, becoming de facto no longer accessible to the public.