I also think mechanics goes a long way to perceived arm strength. You're not gonna "bulk up" a throwing arm to where you can fling it 70 yards, but a good coach can help a QB get the ball to his receivers quicker and more efficiently by tweaking footwork, shoulder/elbow placement, release points etc.
finally. it took years, but someone is on the right track in understanding how this works. and how it gets coached.
bulking can help (a pretty good amount)
mechanics and technique help even more. (usually having to do with footwork and lower body, just as much as arm/shoulder/elbow).