I'm not the person who made this obviously, but I've talked to a lawyer about making derivative fan games before. Their advice was that , unless you have permission, even mentioning the original work at all would infringe on the copyright - which surprised me, my natural instinct was that it should be mentioned as an inspiration, similar to listing sources in an essay.
Obviously I have no idea if that's even 100% true and even less of a clue if that's what happened here, but I'm sure they weren't trying to be deceptive or anything.