Why do disc brakes stop faster in dry conditions?
No, the stopping distance is not just traction limited - or at least not just by the wheel/road surface combination. Even if you can block your wheel with your rim brakes and get the wheel slipped, you might be able to stop faster with better modulation when you actually avoid the slip. The consistency in breaking in the disc brakes helps to predict the exact force necessary.
Also, the raw power is definitely bigger, there is no question about it..