The low detail really is due to the outdated Quake 2 models. You could go quite a bit higher on geometry and texture detail before performance becomes an issue. I actually don't know why Christoph picked Q2 specifically for this project.

The lighting in this version of Q2 is vastly more physically correct than in a lot of current generation modern engines. So it really should feel more real. I think this shows why hardware accelerated ray tracing is such a big deal for realtime graphics.