For the curious, this is based on the https://github.com/FWGS/xash3d-fwgs engine. Probably this fork: https://github.com/icrazyblaze/Xash3D-Emscripten/
https://github.com/FWGS/xash3d-fwgs
https://github.com/FWGS/cs16-client (but this is closed source now, I don't know exactly how they use it, maybe there is some different fork of `cs16-client`)
These two are what their own references point to when you load the game.
No, it seems to contain some leaked SDK and reverse engineered code: https://github.com/FWGS/cs16-client/tree/v1.32 , and to run on https://github.com/FWGS/xash3d-fwgs .