Copyright concerns go beyond being sued for infringement, as @VilsBae just pointed out on the other thread. I agree that under the present state of the law, it’s unlikely that any content creators would succeed in suing the author or HG for infringing copyright. (Though that’s still far from settled and will probably end up with different levels of legal vulnerability in different jurisdictions where CoG/HG sells its work.)
But because AI generated work is currently not copyrightable under US law, defending the game’s copyright by e.g. sending takedown notices to pirate sites that profit off HG work immediately becomes a dubious legal quagmire.
So far the courts haven’t been quick to treat AI as infringement – but nor have they been particularly inclined to treat its fruits as copyrightable. An author who represents AI-generated work as their copyrighted creation is not legally “in the clear,” even if they hadn’t signed a contract asserting that they would specifically not do that.