I've been working with the leaked wipEout source code for a while[1] and I can assure the quality of the Quake source is absolutely stellar in comparison. While Quake's source may not be up to modern best practices, the overall architecture certainly has a lot of structure and thought put into it.
Modifying Quake to run on the Occulus Rift was a breeze[2], compared to the mountains of garbage I have to wade through with wipEout.
[1] https://twitter.com/phoboslab/status/1653707447586922498