It signifies his superior athleticism but youre right a tackle would hardly ever need to run 40 yards. It just shows how explosive he is. That along with other drills gives you an idea of how well a player can move in a short distance.
One thing I would look into is his endurance. I imagine he's mostly made up of explosive fast-twitch muscle fibers so the question would be does he have enough endurance to last on those long 12-15 play drives and remain explosive?
Either way, I hope the Dolphins stay away from offensive line with our first pick. I realize the importance of offensive lineman but I dont trust this staff to make the right evaluations on linemen on a consistent basis.