I think his point is that the astroid is no longer a solid with a core - if correct, then as it enters the atmosphere it will begin to break up and burn easier than had it remained solid. Just a thought.
If it breaks up in the upper atmosphere, it will do less damage than if it stays solid enough to hit the ground, but that's still a lot of energy. Some of that will reach the ground in the form of concussion and heat, and you will have dust in the upper atmosphere affecting the weather for a long time.
The 2013 Chelyabinsk meteor was just 20 meters wide, and released the equivalent energy of a 400-500 kiloton nuke.
Exactly. Would you rather be hit by a ton of feathers dropped from 100 feet, or a ton of concrete in a solid cube??