I think he is referring to method NBA teams select newcomers. They take well trained young person and they have them do vertical jump. Because they are well trained, their vertical jump will not improve much because there is genetic component to it. But if you are average then of course it will improve with time as you do training, for example if you lose weight you can jump higher or if you practice and improve nerve-muscle connection that will increase your power output it will also improve your jump. But at the end of the day there is genetic limit for the most part, but just like with everything there are probably exceptions...
Kinda like IQ test, if you train specifically for it, you can get better. Vertical jump is a good measure of power, same as a IQ test is a good test of intelligence.
