I think it's sort of reasonable in this case, given there's very little investment in running on 4 limbs, while nation states fund track and field programs to the tune of millions of dollars. That the small number of 4 limbed runners are making progress against 2 limbed runners does indicate it might allow higher peak performance. Of course, 4 limbed runners have probably already plucked the low hanging fruit in performance improvements, but the researchers aren't projecting linear improvement

Don't sprinters start with 4 limbs on the ground? The advantage of the 4 limbs seems to be gone by the second step.

Trail runners often run with poles on up or down hills. Not sure about top speed, but from experience the poles definitely help with endurance.