I agree with you but copyrights is en empty threat. Do you really think he officially copyrighted this barely-started little porn game and will sue someone to court? It just the right thing to do for any decent artist not to steal a previous work.
It depends, I suppose, on whatever jurisdiction a case would appear under. In the United States, for instance, you are considered to have a copyright without having officially registered anything. You just have to be able to present enough evidence that supports your claim. It's not very expensive here to register, so I would imagine that in many places it's similarly affordable. And while I agree it's unlikely, I have no idea whether he registered any kind of copyright for this game. Given that some porn sites actually do have their stuff copyrighted, I wouldn't put anything beyond the realm of possibility.
I have seen many times even supposed artists on this site (I have no way to verify whether they're legit) being all for just running away with someone else's work. Given that, I think your "decent artist" statement doesn't really matter. I agree that they definitely
should not; I just don't have much faith that they
would not. In any case, copyright wasn't even the primary point of the argument, but more of an afterthought.