It's a case of definitions. It depends on what they intend to be.So as a follow up to this:
Do these books skip writing the 5e rules altogether, or do they reprint them (with the necessary modifications) for the sake of being thorough?
I'm very curious what the standard formatting for such a book would be.
If they include the game rules, they're a game.
If they don't, they're a setting or a supplement.