There’s nothing illegal about it per se, unless the author is reusing original Summertime Saga content, but if they do, well… The only way to prove that would be to steganographically pack enough data into the original artwork, so that some of it would prevail whatever the copycat is doing with it. When done right, whether the guy’s drawing over it, or training a generative AI model, wouldn’t make a difference - it should work either way. Probably enough to convince Itch to ban them.