Are you sure? I haven't played enough Quake to know either way, but I did play Deus Ex in software mode (unreal engine), and that had a lot of differences to the other renderers. I very much doubt texture sampling mode alone would reach a similar result.

(It even had graphic errors, green textures in the faces etc giving everything a very alien look. But that's not what I mean).

It depends on particular renderer, obviously. Not all software renderers were being given appropriate care.

It's not correct. A notable example is the water effects.