That doesn't make any sense. Based on a skim of the KDP site, it looks like they're using the spotify payment model. In other words, a portion of kindle unlimited subscription proceeds is used to pay authors that have enrolled their books to be available on kindle unlimited. The allocation is determined by some metric (eg. time spent reading book), but the total amount is fixed. It doesn't matter whether amazon offers 1M books or 100k books, or a user reads 1 book or 100 books, since the only thing that changes is how the amount gets distributed among the authors.
I asked someone who was published how Kindle Unlimited works. There's a pool of cash reserved for each month and authors are compensated a certain amount per pages read. There's a maximum number of pages, per customer, which will be counted.
