This is false. Look at the horse's digestive system.. There is a reason a horse CAN (not shouldn't) eat a bag of feed in one sitting. The stomach doesn't get full its the hind gut. Do you really think even 5 lbs (not saying you should feed 5lbs in one sitting either) of unsoaked beet pulp is going to make a difference?
Worked on a farm once where they kept a bag of wheat for the chooks in the horse's tack room. One night a tricky mini horse broke in & got into the wheat. Worked out later that he'd only eaten about 1kg(2 pounds?), but that was enough. We found him dead the next morning. The (rich) owners decided to get an autopsy done & his stomach & hindgut were both ruptured from the expanding grain.