Nope, I'm saying in a normally functioning human being caloric surplus period is what causes gaining weight. The insulin response is something that has gained a lot of popularity recently because people like complex explanations, but the truth is, as I said, it's pretty inconsequential outside of behavioral impact. It's not that it's not involved, it's that you don't need to understand or manipulate it to gain/lose weight.
It gets tricky in a few places: calories are labeled as the number of calories a food has in it, not the number of calories you net when you factor in the cost to the body of extracting those calories. 200 calories of chicken breasts costs your body more calories than 200 calories of sugar to extract. Especially if you don't end up using the protein to build muscle and convert it to energy. This isn't a massive difference, but it's significant. Regardless, calorie counting is about establishing a baseline and trending downward, not being 100% accurate in your calorie estimations.
As you mentioned, people with specific needs, like diabetics, are going to have different experiences with carbs, but that's a completely different discussion, and not one I know a ton about.
The body builders thing is interesting. I'm a power lifter and I lift with a lot of body builders so I am familiar. Insulin use like this can help the body absorb nutrients more quickly after a workout (specifically carbs, which can help restore glycogen to the muscles which helps with amino acid absorption) but it's not being used without an increase in overall calories (or it shouldn't be). In short, it's helping body builders eat more (especially post workout), not absorb the same amount differently. In my opinion it's a really stupid idea, though when you consider the trash many body builders put in their bodies it's probably less scary than a lot of the supplements and prohormones out there.