I think (correct me if you know better) that you can’t dive into the ruck. If he had have scooped through and picked it up and stayed on his feet it would have been legal.
Yeh its in the notes to section 11 of the International Laws of the Game, which deals with the tackle and play the ball. It states as follows:
4. If the player marking the tackled player at the play-the-ball dives behind the tackled player in order to drop on the ball as it is heeled, he is guilty of a voluntary tackle and should be penalised. If there is no acting halfback it is permissible for a player to dive behind the tackled player and drop on the ball after it has been heeled provided that, unless tackled, he immediately regains his feet.
Section 11 otherwise states:
When the ball touches the ground it must be heeled (i.e. backwards) by the tackled player. The ball must not be kicked or heeled by the player marking him. The ball is in play when it has been played backward.
So there was nothing to prevent Moses from running through and picking up the ball.
We may have then been penalised for interfering in the ruck as I recall Robson protesting he couldn’t get through because a player was lying in front of him. However, IMO it was mainly because he was looking at his line to see what he was going to do.