Why are we trying to Devil’s Advocate and defend the art house that’s been caught distributing copied art at least twice?
Generally an art house doesn’t let artists hold individual rights and make decisions for how their art is sold. Part of working for another person as an artist usually involves transferring your rights to that person. The only way you usually negotiate a better deal is to be someone whose art is famous enough the person with money feels they have to tempt you away from other people with money or, worse, starting your own art house based on your talent.
I don’t think the devs’ response indicates they think they’re buying stock art that might be present in other games. I’m willing to bet their commission contract explicitly states they want original art. Licensed art is a different thing entirely.