They way to look at this from a insulin resistance perspective is that years overeating caused them to develop insulin resistance, which in turn caused them to become obese.

And when they are given tools to help them not to overeat like a gastric bypass did their insulin level resistance decrease?

Also on the same show, when they were put under medical supervision where they had to consume only 1200 calories a day they would drop around 30-50 pounds. There was one guy who would lose 20 pounds in a week (he weighed over 800 pounds) when his mom couldn't sneak food to him.

I'm sure they were quite obese before insulin resistance kicked in.

That depends, in some people it's resistance first (essentially prediabetes), in others it's obesity first. Some others develop insulin resistance when even moderately obese, there are a few who never develop it despite being extremely overweight.

The lower insulin resistance, the better the effect of diet, usually.

If that were true, weight loss surgery wouldn’t help. It takes away their ability to eat too much at once by reducing the size of their stomach. - it doesn’t change their insulin levels.

If you watch the show, most of them overeat as a coping mechanism and part of the treatment is psychological help.