Pulling back to any meaningful extent on platform pedals requires exerting enough downward pressure on the pedal at 6 o'clock to create significant friction between sole and pedal. That is wasted energy, since pushing down at 6 o'clock isn't driving the cranks around. And requires wearing a somewhat specific type of shoe, with plenty of tread on the sole, and an appropriate pedal, with a high friction or cage-like surface. That limits your choice of footwear, no leather soled loafers etc.

I'm not saying there aren't ways to pedal more or less efficiently with platform pedals, but I think the potential benefits are quite small. I'd focus more on correct gearing and cadence, leg extension, and body positioning (aero).

When using clipless, you can actually pull up. For short bursts of power, like attacking a couple blocks of hill. But it is hard to sustain. I used to consciously soft-pedal on the downstroke and do all the work on the up-stroke, in hopes of building the "pulling up" muscles, and even did targeted gym exercises. Still couldn't sustain a hard pulling-up effort for very long.

I think your body just isn't designed to pull up. Your legs are designed (evolved, created, whatever) for walking and running, those activities require far, far more pushing down than pulling up. So that's how your muscles work best.