If they were giving for free sure, I wouldnt care too much, but the fact they are selling it and passing it off as their own art would be, for me at least a kind of false advertising or deceit. I think devs/game makers should be transparent on where and how they use AI.
Before AI ive seen some really good art similar way as this. So, Id like for the dev to be honest and tell me themselves if AI was used.