As far as I know saddles are not a requirement either. I can't remember her name right now, but there was a rider that rode many rides bareback. Horses get lame at rides, they get sore sore backs, girth galls, dehydrated, etc, regardless of equipment or supplements we use or how much water we offer them at rides. It can happen on a trail ride or a 25, 50, or 100 mile ride. We have the vets and ourselves to monitor our horses' conditions and stop when we need to. I have a horse that if she were younger would be a candidate for barefoot 50s and even Tevis. She's never taken a tender step.
As Crysta says, these riders were not trying to prove anything, they just wanted to ride Tevis and their horses do all their rides barefoot, including 3-days of 50s, they didn't want to change what works for their horses for Tevis. That's what makes sense. Their horses have a good completion records of doing 50s barefoot and they are accustomed to riding on rocky terrain. It works for them and its not a problem or issue.
Ride managers determines whether hoof protection is required at a ride. Many rides allow barefoot horses to participate. Tevis ride management made a good decision to allow proven barefoot endurance horses to enter the Tevis ride. The proof is in the results.