true, in the official source code they say that was only tested the build in linux lol, and in the first doom they said that was only playable on linux :b
IIRC the guy who was doing the official Linux builds of the old id games was also the one who was publishing the code, so that's probably why.
They could never publish everything they had internally, as they used proprietary or patented bits in places, so the code always needed cleaning up before GPLing it. That's why Doom engine source was released without the sound, the Doom 3 engine had the shadowing code replaced, Quake 3 code didn't have the single-player parts because people were still licencing it in the mid-00's, and other such stuff.
And yes, Doom was originally developed for *nix, I think they were using NeXT machines or something like that, for development.
The license doesn’t really matter, unless you mean it should be included in the kernel. If it was another permissive license, you could also port it to Linux. I have no clue if it’s actually possible to port it to Linux natively, but I would guess that you can