Once you reach a certain level you can bend - no pun intended - the rules a little.
In any case, what really matters is that your shoulder doesnt go too far beyond the knee and the knee doesnt go over the toes, or better, it remains straight up. As long one keeps it there - and is as athletic as those masters of the craft - he can retreat more or less quickly - and often safely as the attack was already lunched from long range. A different story when the knee bends beyond the toes, then you got no leverage all of a sudden, and no matter who much squats and dead lifts you do you cant stop your own inertia at once, that creates a small delay in the return to guard where one is heavily exposed to counter...
So they sink the posture, open the stance while steping ahead and lunge from afar... In reality, as you are sinking, you only gain a few cm but those are the ones that make a difference in the top echelons of martial arts.