What, no it absolutely isnt... It is a requirement ONLY on mastodon.social. All other servers running mastodon have no such restriction.
This person is on mstdn.social (Stux's server) and they have no such obligation. It is perfectly acceptable and allowed to use undisclosed AI generated images on that server as far as I know.
If you doubt it just ask @stux as he will either tell you you are harassing a person that is working within the rules (in which case you might want to apologize) or he will tell you the person is indeed in violationa nd handle it. Either way if this person is using AI generated images or not that in and of itself is a vioaltion.
Now what would be more concerning is your accusation that their **posts** are AI generated and fake. But seeing as they have replied here in a way that doesnt seem like AI, that doesnt seem to be the case. Moreover it is problematic to make these statements to such a large follower base due to the tendency to dog-pile on a person who seems to otherwise be a real person, and even if you are legitimately right that they arent you should handle this with your system admin and the admin of the foreign server due to the potential harm you could cause if you are wrong, and the fact that you lack the tools to know if you are correct.
Just my two cents, I dont think you mean harm, but you should be aware of the very real potential for it.