Correct me if I'm wrong, but if they extrapolated this from one-legged hops, which put all of your weight on a single leg, isn't that more akin to measuring the ground force from jumping than it is to running?
The point is that the limiting factor is not how much force the limb can exert, and that's what was demonstrated by the one-leg hopping.
"Our simple projections indicate that muscle contractile speeds that would allow for maximal or near-maximal forces would permit running speeds of 35 to 40 miles per hour and conceivably faster"
So essentially if human physiology worked differently than it did, we could theoretically run much faster. Not a useless result, but not really anything as exciting as they make it sound.
If you could get all that force into running stance...that's like saying michael jordan has a great vertical. If a runner could apply that same vertical while running top speed, he could leap really far.