"There is no law against placing the
ball twice or three times, as long as the body is not moving
crawling/jumping forward after the momentum has finished.
This is not a double movement because the body itself is in a stationary position.. A double movement is propelling your body along the ground, after the momentum of the dive, has finished."
it has nothing to do with the body "moving crawling/jumping forward" or "propelling your body along the ground, after the momentum of the dive, has finished",...
to begin with there's no such concept in rugby union, ‘double movement’ is a rugby league term and not a phrase associated with the laws of union.There are 2 laws that intervene in this case.
Law 14. Tackle: Players responsability.
14.7. Tackled players must immediately:
a. Make the ball available so that play can continue by releasing, passing
or pushing the ball in any direction except forward. They may place the
ball in any direction.
b. Move away from the ball or get up.
c. Ensure that they do not lie on, over or near the ball to prevent opposition players from gaining possession of it.
Sanction: Penalty.
and Law 21. In-goal. Grounding the ball
21.7. If a tackled player has momentum that carries them into their own in-goal area, they can make a touch down.
21.8. A tackled player near their own goal line may reach out and ground the
ball in in-goal to make a touch down, provided it is done
immediately. Sanction: Penalty.
The key word here is: immediately
The player didn't immediately make the ball avalaible, instead he lied on it and after some seconds (not immediately) reached out and grounded the ball. As the TMO said to the ref it was a penalty against the argentinan player