The theory is about total carbs, which includes sugar, potatoes, etc, not just grains. Strange to just focus on grains.
I thought that the theory was the simple carbs (e.g. sugars, refined grains) were worse offenders than complex carbs (e.g. potatoes).
The "complex carb" and glycemic index thing is pretty well nonsense. A carb is a carb. The only question is whether you have a properly functioning metabolism and insulin sensitivity. They call a screwed up insulin sensitivity "metabolic syndrome". If you've developed one, typically through many years of poor eating, you need to cut out starch, because your fat cells and pancreas can't handle it properly. If you're thin, well then the glycemic index thing is useless. Your pancreas and fat cells work fine.