No, the leaked code wasn't involved, it's all reverse-engineering and Quake's open source code where applicable afaik.
Unfortunately, it was by original Xash3D author. In FWGS we do not take any leaked code, but we also do not tend to do clean room implementations, since it's easier to work with decompiled code.
And license-wise it would be very hard to get rid of the fact that Uncle Mike used some stolen code.
You’re probably thinking of the HL2 engine, which was leaked in ~2003.