The standard that you must meet to get a pilots license is being able to hold your altitude within 100 feet in a 360* turn. A 40 foot
He's not saying the drop was 40 feet instantaneously, he's saying the turbulence and the subsequent recovery only caused a 40 foot deviation from the assigned altitude.
Just for reference, a descent rate on a standard flight is pretty normal at 40 feet per second. Some descent profiles can double that. The NCAA diver will hit the water at 46 feet per second.