As others have said the rule was changed, and the puck going through the cease no longer negates icing. I'm not hung up on it because the Bruins simply should have handled the play better, but I'd just like some clarity on the decision.
If as I've seen said the call was based on the fact that Forbort could have reached the puck before it crossed the goal line and didn't do enough to try and reach it, I find that tough to accept. He probably could have skated a little harder, but he was never reaching that puck in time.
I'm also not sold on that as the reason. If you look at the linesman, it's precisely the point where the puck clips the crease that he waived it off. So either he mistakenly thought it touched Swayman, or he simply didn't realize that the rule has changed. Which sounds tough to believe, but this is the NHL we're talking about so it's well possible.