You don't know the heart rates of any of those riders. You can't tell someone's heart rate by looking at them. And HR varies between individuals- at the same power output two different riders who have the same threshold power will be at different heart rates... maybe 20-30 bpm different. Even if you knew their HRs, it would be useless.
Comparing HR between individuals is no better for understanding performance than comparing their shoe sizes.
He's in the big ring because 1) it's not that steep 2) he's pedalling out of the saddle at a relatively low rpm 3) some riders like to use the big ring because it intimidates the other riders or makes them feel strong.
It's interesting to see how slow their cadence is compared to modern pro stage racers.