Calling a license by anything other than its name and stated purpose is something I’d dare to call mislabeling. If CC BY-NC-SA 4.0 decides to add “anti-commercial-AI” then and only then is it not mislabeling. That’s like me calling the US copyrights of the books sitting next to me “anti-bitfucker” licenses. They have nothing to do with you at this point in time so it is misleading for me to claim otherwise.
While you are correct that lemmy itself does not add a license and many instances do not add a license, it’s not as simple as “the user notifies [you] must abides by [their] licenses.” Jurisdiction matters. The Fediverse host content is pulled from matters. Other myriad factors matter. As you correctly pointed out, there is no precedence for any of this so as I pointed out unless you’re willing to go to court and can prove damages it is actually useless.