Hat tip to Pensburgh reader @bmainou for the screenshot. It looks like you can see the tiniest bit of white behind the puck, meaning it was completely over.
To the official's credit, the Root Sports feed didn't get this shot up until late in the review, and by that point they may have determined that they couldn't tell. Hell, they might even have seen this but still thought the same.
By virtue of this goal not counting, it means that the continuation of the play did count. Unfortunately for the Penguins, that meant that Josef Josefson 's short-handed breakaway goal did count.
Moments later, the Devils would tack on another to take a 3-1 lead. If football is a game of inches, hockey might be a game of millimeters. What could have (should have?) been a 2-1 Pens lead, snowballed out of control the other way. Maybe this is karma for when, in 2010 then Fox Sports Pittsburgh (the precursor to Root Sports) with-held a replay of a goal that would have showed the Flyers score.