Some horses naturally wear their hooves down pretty well, we see that a lot with mustangs. Some horses don't. Part depends on the footing as well and the conformation. These guys were on a soft muddy pasture. Lack of proper nutrition played a part in it as well. Usually in ponies and mini's we see founder more from obesity that skinny. But as this little guys proves, there are exceptions to every rule. Also, they were only eating wild grass and treats. Generally ponies and mini's don't do well on free grass fed diets, and spring grass is higher in calories and protien.
This is my experiences. Others may have a different opinion. Also, I have found once they have foundered they will forever be more prone to it.