monogastrics will happily ferment fermentable fiber in the gut, extracting vitamins, minerals, and short chain fatty acids which provide calories. Beet pulp has a lot of fermentable fibers, and it is normal that it will help chickens in small quantities. it is long fibers, cellulose, that does not help.
Feeding a high fiber to a monogastric animal will not only dilute nutrition but it will also increased the rate of passage through the gastrointestinal tract (GIT) and less time for digestive enzymes to act.