Ben, you've explained one mechanism of fiber and overlooked several others.
Fiber also lowers LDL and ApoB, provides substrates for microbial fermentation, produces short-chain fatty acids such as butyrate, and supports colonic physiology.
None of those benefits requires a carbohydrate-heavy meal.
And while fiber isn't classified as an essential nutrient, that doesn't mean its absence is optimal.
The colon has a physiology beyond being a convenient exit for yesterday's steak.
Fiber is helpful, but helpful isn't the same as essential.
The glucose benefit comes from soluble fiber, which slows how quickly starches and sugar from a meal reaches the blood. Insoluble fiber does little for blood sugar.
So soluble fiber's value for glucose control depends on how much glucose there is to slow down. With carb-heavy meals, it can make a meaningful difference. With low-carb meals, there's less to blunt, and the value mostly depends on whether a person may still want some fiber to keep bowel movements regular.