Hold on there. High fiber consumption increases the excretion of cholesterol, by reducing the reabsorption of the cholesterol in bile. The liver produces cholesterol for bile, which mixes with our food in the duodenum and aids absorption of fats. Most of this cholesterol is then re-absorbed by the small intestines. By increasing bulk, fiber reduces the amount that is re-absorbed.

Effects on but biome are real too, and apparently beneficial, and may factor in, but it isn't the only (or necessarily the primary) mechanism for reducing serum cholesterol.