I was also surprised to see these huge functions with multiple nested layers. Is it common for game development? Or it was just a quick hack on top of Quake engine?
Yes, it's very common in player / vehicle controller code.
People start with a clean model of how the player should move, using minimal physics, then add layer upon layer of special case code for various environmental / state / networking edge-cases.
It's usually what makes the game actually feel good to play.
I've heard looking at the first person movement code for Valve's games will make you wish for your own death, but stripping out the weird stuff which looks ideologically wrong just makes the game play worse.
