At first I thought it was okay, but looking at it further, the puck is 15 feet away from Backes when Seabrook unloads on him. If Backes was in the process of playing the puck, that same hit is legal.
People like to target Boston, LA and Chicago for over-the-edge play. Is it a coincidence they're the last 4 Cup winners? Maybe that fact speaks to what a 2-month war the playoffs really have become.