A lot of it is public domain photos that they pretend they own the copyright for.
Eg: https://www.gettyimages.com/detail/illustration/people-and-t...
This isn't a photo, but there are millions of pics as well.
Getty asserts a related point in the claim- that the work done to build extensive metadata for the content (which Stability apparently used for training their models) is a valuable service in and of itself.
Edit: just clicked through your link above and should have done so before. While the points above still stand (imho), the presentation on the Getty site borders on misrepresentation in the implication that they are copyright holders. Leaves a bad taste in the mouth.