yep, was going to say that this is probably the real source behind the mystery. Early OS X is basically NeXT when Jobs came back to Apple. Doom was developed on NeXT, so id Software and Carmack already had a history of using BSD. They also released Quake for Linux around 1996, which may have used X11 (I cannot recall, maybe it just depended on 3dfx/nvidia without X11 at the time... it's been so long)