@steffo well, not literally, but...
My assumption is that Ritual received Quake 3 source code at the end of 1999, not too long before it's release. fakk2-sdk hints at it as all files have short changelog at the beginning with the exact dates. The engine functions also appear to be different from Quake 3 Arena source code, but that's probably because Q3A had tons of updates before the source code release in 2005. It also has random Quake 2 (?) leftovers.