The author claimed that he is using a vanilla engine (which is GPL) and proprietary artwork and (scripted) game logic, which would be OK, as artwork does not constitute a derivative work.
That was my impression as well. I do not think scripted game logic affects the GPL either way. I know for a fact one of the reasons the Steel Storm developers picked DarkPlaces is they liked its scripting language, which is also why they are not really using their Unigine license as they do not have the same fondness for the one it uses.
For example, if I make a program that requires a free software runtime environment, but that program itself is proprietary, does that constitute a violation? As far as I know, that is not the case. So I think they are in the legal clear.
I think the main question is whether someone could reasonably (in principle) swap in a different implementation of the runtime (a la POSIX, libc, sh, Java, etc.). If so, there is no "requires a free software [or rather GPL] runtime environment"; it's just "requires some compatible runtime environment".
Hurray. But this announcement makes me feel very old. I remember all the years I waited for this game. And now the engine is considered to be of no further commercial value. That and the "Big 5-oh" looming less than 2 years away, makes me very happy for the rest of you. But excuse me while I go cry into my pillow. ;-)
Hey, seconded. Yes I was ten when it was released, but still, seconded.
Just wait 30 years. And you'll get yours! ;-)
Seriously, though. Enjoy the next 30 years. Being 48 is cool and all. But one of the things you learn is that you can never go back. Partially because the world changes. But mostly because you change. Which is fine. And usually an improvement. But the lack of a rewind button does become increasingly noticeable.