Fiber in flours

So the goal of no sugar no flour is to bring down our insulin. We want to eliminate flour because it is a concentrated food which would then raise our insulin. But fiber slows down the insulin response and whole wheat flour has a lot of fiber. (To be clear, I’m talking about homemade breads, not store bought because those are processed to the hilt.) I guess I’m having a hard time understanding how whole wheat flour raises insulin.
I understand this is all up to me and I can experiment, but I want to understand the science and logic behind it. Especially to offer my clients better understanding.