my understanding is that the harder you are exercising, the more blood gets diverted from your stomach to your muscles, thus making digesting more difficult.

Therefore, it might be ok to eat a certain amount of fibers during lower intensity riding, but if you are pushing hard for 3 hours, the digestive system will change accordingly, and the same amount of fibers might cause problems. Thats probably why gels are preferred for fast digestion, especially during hard efforts.