So what prevents uke from cutting the leg? In the video posted, uke often has a clear line to the leg (see 0:42) and less distance to cover than shidachi does to any target available to him.
I believe the answer to your question it is the pressure on uke sword,
This is the same idea as winden in medieval fencing.
If uke were to attack the leg, just continue the pressure with a forward motion that will deflect and shorten the strike and open the body,
As well that pressure will create enough time for nage can pass the exposed leg back to give to his attack