I'm probably being naive but I think imma believe the dev for now. I think it's just a combination of a natural sketchy art style, with a very unfortunate oversight. Arstyle is too consistent between poses, expressions, and outfits for an AI. Also neither eys nor ears have the natural shit that comes with it.
Biggest tell for me is the fact that the dev mistakenly left some references in the image folder. You can check it yourself if you use the unren.py thing, you can see the author left some references there mistakenly. Face and poses are the same, which you cannot get with ai in any sort of reliable manner.
So yeah. I don't believe its ai