It doesn't matter if the ball is moving at 30 MPH and the racket is stationary or if the ball is stationary and the racket is moving at 30 MPH or any other combination. The experiment did not preclude attaching mass to the handle creating and effect. A stiffer racket and strings would shorten the round trip so there could be an effect. That experiment is simple enough for anyone to duplicate if you want to try different setups. I would be interested to see if the is an effect with a hard mounted handle.Now what happens if they are moving forward, i.e. "transferring their weight" into the shot?
What about hitting volleys with your legs instead of swinging? These are all things where we tell our students to get their body into the shot. Some of these "such and such is a myth" threads are based on some misunderstanding of what the advice is for.