To clarify TSR, if the backline creeps up in front of the back feet at a ruck, they are most often penalised even though they have no contact with an opposition player. What is the difference with Smith? He may have come from onside but until the ruck is over in my opinion he becomes offside as soon as he moves in front of the back feet. The fact he is cleaning out an opposition player near or attached to the ruck should be immaterial.
I have no issue with the Reds winning the game, and if, as you say, they were playing under advantage, then they were destined to win anyway. I just have issues with what Smith did at that ruck and with it being not reviewed or ruled on.