I can't agree with him either. When I have the misfortune to ride on flat pedals, my foot comes off the pedal on the upstroke until I slow my cadence down enough to compensate. Varying the position of your foot on the pedal varies the load on the individual quad muscles, but without a way to attach your foot to the pedal securely, you don't use your hamstrings very much. I love using my hamstring muscles when cycling - it gives my quads a little rest and gives me a little extra speed.