There are improvements he can make in ball velocity by strengthening his lower body/core. Throwing a football is a core to extremity movement, and everything before the shoulder is about generating power. After that it's about direction until you get to the wrist, and then it's about spin and placement. Same thing as a free throw. Shoot a freethrow without moving our wrist (shaq because he broke it) and it'll just be a can of corn. Shoot a freethrow without your legs and it won't go anywhere.
At the end of the day mass move mass. It's why weight lifting has classes. It's a give and take with mobility and flexibility of course, but with Kyle i think he could put on 15 pounds and not lose a lot. It has less to do with him improving arm strength and more to do with him deadlift and squatting, but there are people paid way more for that than me and he has access to them. Get ready to eat some cheeseburgers.
Now all that said there are some freaks of nature. Vick had an arm cannon and he weighed 185 soaking wet. Raise your right hand so that your elbow is at a right angle and your hand is straight in the air. Now keeping your elbow and shoulder where they arm, move your forearm back as far you can. Probably won't be able to get it very far.
somehow vick could go into that position and move his forearm back by like 70 degrees. Just an insane ability to generate torque.
And then there's just differences in muscle insertion points and muscle density etc that we won't know unless we cut them apart and biopsy.
tl;dr kyle needs to deadlift, squat, and eat sandwiches. some people are freaks and life's not fair. anyone stronger than me is on steroids.
This is how it works:
The vast majority of NFL fans want to focus on football. A small, loud minority of politically motivated people bring up Colin Kaepernick, a person who's been a political figure for the last three years. When the real football fans kick back at the political talk, the small, motivated, loud political people accuse everyone else of not only being political, but being intolerant, bigoted, and so on. It's like dealing with a crazy girlfriend.