They should be able to get fairly accurate numbers from old videos and films of baseball games by using precision timers of the release to home plate time since the distance is known.
I wouldn’t be surprised to find that 100+ mph fastballs were common ................
Not at the standard frame rate of 24 fps. My calcs show a 100 mph fastball travels 146.7 feet per second, and the distance from the pitcher’s mound to home plate is 60.5 feet. That means the ball will travel that distance is 0.409 sec or 9.82 frames. The frame rate is not high enough.