Wow haven't been here in a while. Interesting stuff going on.
For the first two, the big white guy, his terrible form is more a product of his low vertical. He does not have the reactive strength to perform a tuck jump with good form and enough hangtime.
For the ankle hops - he is not overloading his ankles as much as he could if he did not flex knee.
For leg press - he's not training for strength; he is lifting very slowly, stopping at bottom of motion wastes the kinetic energy that can be turned into an explosive lift. That type of lifting is good for endurance and general strength/hypertrophy maybe.
Lol I loved this thread by the way, worst depth jump I've ever seen. I felt lazier watching it