If you wear a glucose monitor and experiment a little, you'll find that the composition and timing of meals matter a lot. Eat high glycemic index stuff on its own: big spike, big crash. Eat it with slowly digesting stuff: hardly a spike. It makes sense if you mentally trace how the food you ate will be moving through your digestive system.
French people eat lots of bread, but it's not like American white 'bread' and it is often eaten together with plenty of cheese, olives, butter, etc. From your linked study: "Dietary carbohydrate content may not fully represent glycemic response, because other aspects of the diet, such as fat content and cooking methods, can also influence glycemic response."
Said otherwise: glycemic index != glycemic response.
> The theory about "nutrient-poor environments" doesn't really fit the case of France which is consuming 3x more bread per capita than US while having 4x less obese people.
I think you're misinterpreting. The nutrient-rich French (or American) environment doesn't cause glycemic spikes, but those spikes are problematic when they occur due to that environment. If there were only a very limited source of food in the US, the spikes wouldn't be as problematic (with regard to obesity, not insulin insensitivity).