Strewth. I just rewatched it a few times, the rules state A ruck is formed when at least one player from each team are in CONTACT, on their feet and over the ball which is on the ground.(my caps).
My issue with this penalty is that Potter was over the ball but not in contact with an opposition player. Therefore, the ruck was not formed and he was free to pilfer. I did see the hand on the ground but hardly supporting his weight more an act of balance, many would let that go. But a pedant wouldn't.
He wasn't on the ball before the All Blacks player arrived and bound onto him though. Potter needed to be on the ball before that happened.
His first move is hands on the ground (illegal) and the swipe to pull the ball back doesn't happen until the All Blacks player is in contact with him (also illegal).
It was fine margins but likewise, if you go for a pilfer 5m out from your own line when under a lot of pressure, you need to get it exactly right otherwise a yellow card is probable.