That might be true for sports or leisure riding where you’re always putting in quite a lot of effort anyway. Not so true of utility cycling where you’re not trying to constantly be at your maximum sustainable power.
I live in a hilly place, and hills are harder than the flat mainly because I’m not riding always at maximum power. My heart rate is only about 130 bpm on the flat, and I get a perfectly acceptable speed at that effort. I put in extra effort for the hills because going up the local hills at that level of effort would be much too slow, so my heart rate normally increases to about 160 bpm on the steeper hills.
Except in all those situations where this is absolutely not the case (says science). Even in the first few shots of the video, you can see the guy out of the saddle in his (presumably) easiest gear, because it’s too steep to sit and spin at 150W. Because obviously.
That was much more interesting than I expected it to be. I probably won’t get as scientific when I choose my next bike, but it was very informative. Thanks.