It doesn't really matter. The article suggests that they are selecting and tracking books by ISBN, which means: books that have been published or at least reprinted in the last 50 years or so. And they are trying to get as much of that set as they can, regardless of quantity, quality, or any other consideration. Which means that older books printed before ISBNs became common may be relatively safe, at least from Amazon. And some books just don't come up on the used market very often.

(small historical irony: when Amazon first started selling books, they used the Books in Print database, which included a lot of books not actually in print.)