That statement is politics but what they mean is the effect on girls at 8,10, 12 learning to pitch. I have seen a leaper get a back injury during a game (our opponent), and also heard of other injuries from not being on the ground when the ball is released. You have to figure in the mistakes while learning to pitch. Also, men's FP has been rogue and tiny for decades, they made rules to help build numbers and make it interesting. Not many boys are learning to pitch in men's FP, mostly adults.
To be completely up front here. My DD is a reformed leaper. Yep from ages 9-15 she leaped on almost every pitch. And she got called for it about once every 10 games. Most of the time the leap was hidden by the fact that there was a rut dug in front of the rubber. But once she began starting for the HS it got called more and the winter between Sophomore and Junior year the leap was corrected. I think she has worn out more cleats in the last 2 seasons then she did the previous 6. And for more backstory, she hurdled in 7th and 8th grade. Never a back problem. Now that she is a reformed leaper, she has to go to a chiropractor in season to keep the pelvis in line and manage the pain to a tolerable level. Is it because she doesn't leap? I have no idea, and neither do you know for sure if a girl in a game you were at hurt her back because she was leaping.