That's an assumption. Depending on the incentives in play, the relative scale at which AI users and real humans are affected may well be the opposite of what you expect.
No, that's based on my experience running subscribers-only instances on the Fediverse (Mastodon, Lemmy, Matrix, Funkwhale) for more than 6 years now. I might not have that many users on the servers, but it's certainly a higher number than the zero spammers I ever got.
On completely different scales. Even if it not perfect, it is strong enough of a filter to turn a bot infestation into a mild annoyance.