Yeah, I have asked about the media license in their forums years ago, shortly after 1.0 came out, and yes, it was -NC and from what I have heared it already seemed impossible to change at the time. Becaue many artists were involved and they have (mostly) disappaered even back then (but I'm not sure if they actually disappeared). And today it's probably even “more impossible”, if you know what I mean. You know the drill: Basically convince all artists to relicense their work without the -NC. Yeah, good luck with that.
The real frustrating part? Before 1.0, they had non-free media before, but with a copyright they didn't own. Or it was the engine used media from another game and so they could only distribute the engine, without the media. IIRC, similar situation like with early OpenTTD. So they recreated ALL the artwork so they can have the copyright. Sadly, they decided for -NC. That's so extremely frustrating.
I wish I would have heard of the project earlier, when they started to replace all the media so I could talk to them about licensing stuff. But I came too late. Far too late.
AFAIK the engine alone is under a free software license, however. But that is, of course, not the full game, so The Dark Mod, sadly, cannot qualify as free software.
Basically, to “liberate” The Dark Mod, someone has to recreate all the artwork … ONCE AGAIN. xD Yeah, copyright is !!!FUN!!!