You'd be surprised. – Trump

At around 10:40 in this body language video, the following words are exchanged. I think when Trump says "You'd be surprised." he is telling us that his interruptions are a strategy…or else he would let Joe talk because he is perfectly in control of himself.

Wallace: Mr. President, will you let him (Biden) finish?

Biden: He doesn't know how to do that.

POTUS: You'd be surprised.