Of course football is played on grass (or artificial grass) in cleats, but my guess is that if you had a group football players do two sprints in randomized fashion, one on a track in spikes, and one on grass in cleats, while the grass/cleat time would probably be slower the rank order of the participants would be the same. Fast is fast, if you are testing the same skill on different surfaces.
Does great linear speed make someone a great football player? Certainly not. Football requires change of direction and reaction among many other skills and abilities, but I don't know how a "track" sprint does them a "disservice", just inform them that linear sprinting, on a track, in spikes, with hand timing (if its hand timed) is not a good predictor of football playing ability.
This would certainly affect his recruitability if it was a high school kid looking to go to college. With speed being such a premium nowadays, the difference in a few tenths of a second written on an index card could be the difference between getting a phone call and being sent a 'thank you for your interest' card.
"Does great linear speed make someone a great football player?' Of course not, but its sure as hell is a great place to start.
'Speed is Speed, if your fast you will run fast on any surface' Well said.