Yes Quake 3 is indeed doing something very different. It doesnt use the kind of geometry that A6 uses. It uses some kind of higher-order surface that is tesselated by the App. Other than that i dont know...

The World geometry in A6 is outdated, everyone knows this.. hopefully soon Octree Will be implimented and you can use any mesh you want. or you can just do what i do and use models and polygon collsion.