How is this possibly true? I've increased my vertical jump by nearly two entire feet through years of training in volleyball. Everyone can increase their vertical just like they can increase their squat. It's a full body compound movement.
Speed and power is entirely genetic. you're either fast or not. you can either jump high or you can't. you can improve it but only by slim margins.
Strength however, can be trained and improved greatly and well into middle age, which is why barbell training is so popular.
I played volleyball recreationally for years and the jumping with arm swing became muscle memory. Last week I was teaching my 10 yo son some volleyball basics and I realized how a few extra movements can impove one's jump (specifically the singing and its timing).