True. Consider if a corner gives a receiver a 7 yrd cushion, it takes a very large difference in 40 time for that receiver to beat the corner to a spot on a 40 yrd straight line sprint. Ability to change direction is just as important, as in practice that is how most receivers get open most of the time. And likewise for defenders trying to stay with them but even worse because they have to react and then follow. And all that gets trickier to hang numbers on. It's great if a defender has the speed to run down a ball carrier from behind, but once you get to that you've usually already given up a big play!
Now it *usually* holds that if a guy can do one thing fast he can do other things fast, but it's not necessarily going to be a one-to-one correlation.