In case you were unaware, this is actually the source code of the rerelease of Quake II. The source code for the original Quake II has been released for many years[0], along with many of the id Software classics[1].
In case you were unaware, the source code of the original is included along with the re-release in this repository, as stated in the first sentence of the readme.
If it's anything like the Quake re-release, it's not actually using idTech 2 at all.