Consider the designer drug argument.
Legislators can only make a specific chemical compound illegal. If you swap out a few ingredients, you have a new drug that has to be specifically legislated against to control.
Nah the law on drugs has changed a lot in the last twenty years, so you see a lot more successful cases or settlements from one pharma comp suing another over a biosimilar infringing upon their biologic drug. You even get successful cases where one pharma comp sues another successfully, even though the biologic molecule in question is so complex neither company is aware of its exact composition or chirality. Or the very common case where the court has trouble coming to a certain ruling, because the companies involved in the case are the only ones in the world who know how to make their biologic drug, and they don't want to reveal the exact process to manufacture it, in which case how do you determine if someone has infringed upon another's patented drug? Answer: you just do a settlement to avoid eating away all your money in pointless legal fees
What if AI art becomes the same? Is it a SM? We know them by their pauldrons and the Aquila. What if an AI changes both sufficiently that it can be argued it's no longer GWS classic Space Marine look but something different?
It's not about the stuff the AI makes. It's about the AI itself. I mean GW already goes after people who make "angry space soldiers" and "exploding space bug spores," and they have tried to copyright things like "space marines, lasguns," and all of warhammer fantasy got killed because they realised you can't copyright the Holy Roman Empire. Orks became Orruks, guardsmen became Astra Militarum Soldarium Maximus Quintus Epictetus and other brilliant copyright friendly names, like all space marines becoming Primaris Adeptus Astartes led by the brilliant Chapter Overmaster Biggus Dickus. Anyone remember the lawsuit GW lost when another company was making minis for GW codex units, that GW did not produce a model for? It directly led to so much of the tyranid codex getting cut, tanking the GW share price for a month.
Those are other issuses though. This isn't like Starcraft vs 40k, where Blizard were making a 40k game but then converted it into their own original IP. The argument here is that the AI is itself a commercial product which has been produced using GW copyright protected IP. This is a similar argument that artists have made regarding AI trained on their art without permission or license. Most are not claiming or focusing on the art produced itself, but rather on the fact that someone else is profiting from licensing commercial software that only functions because it used their work for free