It's not that they are more difficult to absorb but rather fiber blunts the insulin response. Insulin is a fat-storing hormone so the less that gets secreted, the less fat you are prone to storing. Coupling that with copious amounts of readily available carbs + fat and you're gonna have a bad time.

But that would mean taking fiber would lead to high blood sugar, and cause damage to your internals. I think the idea that fiber slows carb digestion makes more sense. With slowed carb digestion, less insulin is needed, thus the blunt response.

Not sure how you deduced that fiber would lead to high blood sugar from my response but what I was trying to convey was that fiber "slows down" carb absorption, thereby blunting the blood sugar increase which then as a corollary blunts the insulin response.

This is why most keto diets specify net carbs, which subtracts fiber out of the carb total.