Very interesting, thank you. I'm not really sure how to interpret the charts, but I did watch the video. It shows that clipless had lower felt effort, lower heart rate, but more oxygen consumption for the same speed. It's a bit perplexing how clipless can have both a lower heart rate and a higher oxygen consumption. Does this mean clipless allows the body to burn more energy at lower heart rate? To me that sounds like a massive advantage, but I'm not an exercise scientist. They also point out that the experiment was on a constant speed treadmill, where as in real cycling you're constantly accelerating and decelerating, so the pulling is needed more.
Also I should point out that everyone uses clipless in pro cycling. This is big money sport with big funding for science research. If flat pedals were even 0.1% more efficient than clipless, they'd all be riding flats. But they are not. Based on this alone the question is settled as far as I'm concerned. Is there a conspiracy to push clipless in pro cycling to sell products? No, because if clipless didn't exist they could make just as much money marketing high end carbon whatever flat pedals with carbon whatever shoes. It's all the same bling bling BS no matter what form of technology.
I agree that the pro racing universiality is a pretty damn good indicator that it's the superior technology for racing, but I'm not sure it's clear why. There could easily be some subtle ways in which the clipping in improves certain aspects of riding in ways that were difficult if not impossible to duplicate in a lab setting--transitions between positions, bike handling issues, etc.
I think it's also quite possible that any of these advantages might not be significant in a non-racing setting.