Yes. It was that leaked version, not the officially released code. Fortunately Id Software was very cool about the leak, looks like they've even accepted patches :)

""" wikipedia John Carmack ... and the Doom source code in 1997. When the source code to Quake was leaked and circulated among the Quake community underground in 1996, a programmer unaffiliated with Id Software used it to port Quake to Linux, and subsequently sent the patches to Carmack. Instead of pursuing legal action, Id Software, at Carmack's behest, used the patches as the foundation for a company-sanctioned Linux port.[citation needed] Id Software has since publicly released the source code to Quake, Quake 2....