Empirical data would certainly show otherwise. Were this not the case how would you explain the higher velocity from Barnhill when she aggressively replants versus when she attempts to appear legal?
When she replants she "comes out of her shoes" while throwing the pitch. When she does not replant she is more under control trying to hit her spot. A player should be able to get more push off the plate than they do flat footed on the ground, otherwise what would be the purpose of the plate. My DD can pickup an extra MPH or two if she does not care where the pitch is going and just throws it as hard as she can. You can also hear a difference in the "grunt" she uses when she cranks it up.