Dethrace is just the game engine. It needs maps, car models, textures, sounds, etc. from the orginal Carmageddon game, which is not Public Domain.

That’s not the issue. If the source code is in the public domain, you’re not free to impose additional licensing conditions like “no commercial use”. It’s either released into the public domain or under some license that prohibits commercial usage: it can’t be both.

The art artifacts used by the engine are not part of the source code, and there is no obligation from the game engine to provide you the artifacts. You can either used those provided by the original game (which is licensed under a difference license) or you are free to produce your own and you can release this under CC0 if you wish, but this is anyway independent from the source code. This is a very common practice used by open source games for years.

You’re completely missing the point. We’re not talking about the art assets at all. This issue is that the text (about the source code, not about the art) is contradictory: First it says the code is “Public Domain”, but then it says the code “is for non-commerical use only”. But this is a contradiction. If the code is public domain you can’t impose any restrictions, which is what “for non-commerical use only” is.

My mistake. I misunderstood the question.