I agree that he's not terrible at in-zone defending. But his transition defense isn't good, which is a death knell for defensemen in the modern NHL. He will absolutely need to improve at it.
And his penchant for wildly activating at every opportunity causes his fair share of rush opportunities for his teammates to deal with.
Which is why I think he is the opposite: High-ceiling, low floor. His ceiling is a high-end puck-moving + high-scoring defenseman. His floor, if he can't defend the rush, is not even in the NHL.