Well, it is posted as a pronation drill.
I do agree with you that pronation is a naturally occurring thing when most people throw. I think the drill is to really reenforce the feeling of pronating and just as importantly to get the body, the body rotation, into the act of deceleration to complement pronation.
The video is of the first part of the drill. The second part the thrower would come up on one leg. So, it would seem the things covered with these drills would be pronation, torso rotation as a part of deceleration, also, a firm front leg and learning to rotate around the front leg instead of falling off to the side.
Would like the OP to post video of the second part of the drill if he does that as well.