Eh, the mech.E's I drink with would greatly disagree. if you're modeling your leg as only being capable of applying power to the cranks on each leg's downstroke, then yes, the pedal type wouldn't matter.

but any biomed will tell you that your leg movement is a function of 2 muscles. one to contract and one to relax. we take the "relaxing" (or, iirc,the antagonist muscle) for granted, but where the 1st model describes a 2-cylinder system where inertia (or the pedal) carries the cylinder not performing work back to its top dead center postion, a second, more detailed model would describe the antagonist muscles as a second set of (albeit often weaker, but able to be developed) cylinders capable of producing work on each leg's upstroke.

its a misunderstanding among a lot of people to think that a leg on its return-stroke (or pedal pulling) will provide as much power as on the downstroke. you're probably looking at more like 80/20. but since that 20 is being completely ignored on a platform pedal, clipless can help.

I will add that from my experience, that 20% retraction is almost completely negated by inertia on a fixed gear once your cadence breaks about 80rpms...IME clipless on an FG is more about using your legs as a form of engine braking and/or keeping the inertia of the system from pulling the pedals right out from under your feet.