Two possibilities. There are two schools of thought: dietary fiber acts like a microscopic sponge. As such it 1) absorbs or binds to dietary fat preventing it from getting absorbed in our intestines, and, 2) dietary fat absorbs water keeping our bowel movements soft, which quickens the transit time so that fat is out of our intestines faster.
Fats do not have to . Be harmful. There are good fats and there are some less good fats. Obesity is not caused by consuming fats. Obesity is consuming too many calories and most often, carbohydrates. The body requires that and protein were proper nutrition and only a modest amount of carbohydrates to avoid ketosis. Overconsumption of any foods leads to obesity. Fiber does not protect against obesity.