"shill" implies payment, Lemmy is just way too under the radar for that kind of engagement
And by the time you have shills the network will be drowned in LLMs already. The latter hasn't happened, it's pure paranoia to think the former is anything to be concerned with.
The issue isn't necessarily that the work could be considered derivative, but rather that the notion of copyright exists only for work authored by a human. If it's not produced by a human, then the copyright belongs to no one, and no one can license it because no one owns it.