The addition of fats, fiber and vinegar to a meal slows down the rate of digestion and absorption.
Fats do this by slowing down the rate that stomach contents move into the small intestine.
Thus glucose is absorbed at a slower rate resulting in a blunted insulin spike.
Refined carbs are stripped of fats and fiber and are notorious for spiking insulin due to their fast digestion times.
This is why I don't put too much faith in the glycemic index.
As soon as you add other macros to a meal the GI number changes.
The article attached below is a good read on this subject.
Understanding Digestion: Why You Should Eat Carbohydrates With Fat, Fibre, and Vinegar