Look at the video at 1:10. Benn had him beat. Robertson had attempted to sweep the puck off of his stick, missed and was dead in his tracks. He was lucky his momentum allowed him to swing his stick back around and hit the puck.Robertson is not a defenseman. He's a forward and as a forward he made an excellent play.
He took the pass away without sprawling and stayed on this feet.
He forced Been to make a lateral move which allowed Matthews to catch him from behind.
Technically Benn didn't beat Robertson since it was Robertson who took the puck off of Benn for the turnover.
If he was beat the puck would have been out of reach.
I am happy it happened as I was rooting for the leafs, but it was pure luck he broke up the play.