The definitions part of 20.3 before which does require "whole arm from hand to shoulder to grasp the team-mate’s body "
I don't see a standing FR player losing hand to shoulder contact with their team mate typically.
Meanwhile
(c) Binding by loose head props. A loose head prop must bind on the opposing tight head prop by placing the le20.3ft arm inside the right arm of the tight head and gripping the tight head prop’s jersey on the back or side. The loose head prop must not grip the chest, arm, sleeve or collar of the opposition tight head prop. The loose head prop must not exert any downward pressure.
[and similar for TH].
Again, I don't really see standing props losing this bind element on their oppo ie on a shirt, on the back or side. No mention I can find (somebody find it for me?) requiring hand to shoulder contact etc?
So frankly 20.3 is being shoehorned into practise as a way to penalise standing up. That's not to say sides should be permitted to stand. But its all a bit... "convenient".
didds
There is also the matter of 20.1g where the props must remain bound to their opposition. When 'standing up' this bind tends to be released.