A good defenseman in the NHL would not typically stop skating and try and block a shot in the slot, they would step up and physically engage the player. Especially when they have other players coming back in support from his own team. Marner's stick handling is unreal on this goal, but Dunn gets torched because he stops skating and puck watches. The forward coming back is hardly skating and barely engages him as well, he'd get stapled to the bench in the NHL for that.
It's a great highlight reel OHL goal that really shows off Marner's puck skills, but it can't be denied that there is far more time and space in juniors than the NHL.