Let me be even more specific.

• With the possible exception of a small cadre of elite sprinters: Everyone exerts some negative forces on the upstroke.
• This is (presumably) because you are exerting power to lift your leg, and can't lift it fast enough to completely get out of the way.
• As such, a small amount of power on the downstroke is wasted lifting the other leg.
• You do benefit from lifting your leg, as that maximizes power going to the drivetrain.
• You are not actually adding power to the drivetrain on the upstroke.

And again, without a pedal-based power meter, there is no way to know what is actually happening when you're pulling up. You could be exerting a very small amount of force and feel like you are tugging on the pedals. Or, you could just not be noticing how you're exerting negative force at the bottom of the pedal stroke and going to zero or a very small positive at the 12:00 position. Without objective measurement, you just don't know.

And while I agree that I haven't seen specific data on this: As long as you stay in control, I see few reasons why platforms would be all that much different. The main exceptions are high-power high-cadence situations, where maintaining contact/control would be extremely difficult without foot retention. And even then, it's not that "the upstroke provides power." It's that "being clipped in makes it possible to get your leg out of the way without losing contact/control."

This is not an anti-clipless argument, though. It simply rejects the belief that you gain huge power advantages (especially in ordinary cycling conditions) because of foot retention, as quite a few people seem to believe.

Is that clearer?