IMO the referee and even the TMO got a few things wrong in that match, but that challenge by Haskell was not one of them.
Definite penalty and having committed that penalty, you are then wide open to interpretation for a yellow card.
Haskell didn't have to use a straight arm tackle you might use on a prop, as Murray is not too wide for a clamp tackle, although the ball had gone when Haskell tackled him.
So whether the ref. used a monitor or not, it was still a penalty. I agree with him that a monitor could be used in future but only if referees think its necessary.