I don't know where you were looking but horned frogs do get MBD. Check out Amphibian Medicine and Captive Husbandry, Krieger Press for some discussion on it.
MBD as a term is confusing as there are multiple different conditions that are called MBD (Metabolic Bone Disease). This can result from insufficient calcium, an improper calcium to phosphorus ratio, or a improper ratio of vitamin A to D3. (these are some of the most common causes of MBD).
While you can correct the condition with the support of a vet, any deformities that have occured will remain.
Ed