Tennis is a sport I know well. In tennis you elevate to get an angle to drive low to the net. You don't drive by jumping and your lead foot doesn't leave the ground until you have hit the ball. That is to get into the court. There is no analogy to using a racket. Any sport that uses the arm to throw requires a stable lower body. Like throwing a football, discuss etc.
Keep teaching what pro pitching says is wrong.
Maybe you guys subscribe to the happy Gilmor school of golf.