Ethics of the use of generative AI in the first place aside, I'm pretty sure the illustrator was aware of what they were intending to do with their work (they even were interviewed about it in the behind the scenes video)
I view this in the same way I view the use of an actor's voice for ai generations. Even if the person knows what you're doing with their data, it still feels really scummy and unethical. The idea that we can sample someone else's labor and be able to own that and generate shit from it in perpetuity (probably without paying them) feels very alienating.