See that's where I have the problem, when the puck is on his stick, he can do things against junior players because he's talented but a lot of things he does, there are better options available to him and he will choose the high skill play every time even though the success rate will go less and less as he goes up ranks.
I wouldn't say Smith turns off without the puck, I think that is getting beat into the ground, he had free license in the NTDP to fly the zone early. But in the U18s, I didn't see any of those problems. He came back, he's a stick first defender, very adept at stealing pucks, and he would go to the front of the net and score from in tight.
Smith's high IQ comes from the fact that everything he does has a purpose. He is always deceiving and manipulating, he is reading the entire defense, and making plays off of it and the motion of his team. You don't see that same level of orchestration with Musty, he is only thinking about the play directly ahead of him.