That is the one PP shot he got last night, though this wasn't one of the particularly noteworthy plays the unit had, or things Marner did, and your description of what happened is pretty inaccurate.
Rielly passes to Matthews, who does a rather slow no look back-pass back to Rielly. Rielly collects the pass on his backhand, and then does a forehand pass off to Marner, who is not on his one-time side and receives it down low. The goalie commits hard to Marner's side, and an opposition player is sliding through the crease to block off much of the far side of the net and any lane through the crease.
Marner delays to see if the goalie, who was a bit off balance, opens anything up, or if the sliding player open up any lanes, but nothing materializes and the player behind him is rushing to put a stick in, so he takes the shot, which while not particularly high percentage, is really the only option available and is well placed just below the arm.