Who knows just my gut feeling.

I thought the same at the time

You're asking the tackled player to take a risk though. Does he put the ball down amidst the bodies, try to play it and hope the referee gives the penalty, or does he just step to one side or forward a pace to get a clean play the ball and allow his dummy half to get out with the markers still out of position?

I am not sure what you were watching with that particular play the ball. He didn't just take a yard. He crawled forward first to create the situation where a Leeds player is behind, then he stepped further forward when he got to his feet. It was clear cheating from the player in question to gain an advantage. Rightly penalised.

Good coaching practice is to teach your team to not get tied up at marker, the 'tight 6' markers and As and Bs work as a unit with markers typically splitting 1st marker usually nominating to go the direction closest to their usual defensive position. Standing too close to the PTB at 1st marker is poor practice as AG says.

Yep. There was a habit a year or so ago of markers standing too far back from the play the ball to the point of being only a couple of metres ahead of the defensive line (IIRC on rare occasions the ref penalised them) but it was for exactly the reason you state, to work together with the A's and B's. And now most markers will take a step or 2 backwards immediately after the play the ball in order to close the diagonal gap between them and the A defender.
Usually being tight at marker makes you easier to run past either by the acting half scoot or a quick play to a tight runner.
It's partly why the player playing the ball moves forward or to the side, in order to try and lock in a marker, either tight or directionally.