People are ‘blatantly stealing my work,’ AI artist complains - with notes on a similar appeal that failed hard just recently

The image gatcha does not create a new copyright. There might be a copyright in the text of a complex prompt (do you feel lucky in court?) Mere "sweat of the brow" does not generate a new copyright in the US, so e.g. retouching work on a photo does not generate a new copyright and photos of a public domain artwork do not create a new copyright.

This doesn't touch on the old copyrights of the stuff Midjourney trained on to make its computer-mediated collages. Those copyrights still exist.

Does the computer-mediated collage launder the previous copyrights? The answer is "do you feel lucky in court?"

so if he trained the model himself, would that make the work copyrightable?

I think if he "trained" the model on art he himself created you might have an argument.