My sense is that initially it is the latter, repair, and maybe at a later stage neurons are also formed. The success for DM is better when the disease is not too advanced, I am speculating that there are enough neurons to repair. The other dog that had his treatment over a year ago is still doing well (4the dog was worse than Riley), so the effect of the treatment is lasting.
The only way to know for sure is to kill the dog and analyze the spinal cord. Obviously no dog owner will allow that and no responsible veterinarian would do it.
I think that the attitude of the community has to change and they have to start accepting proof based on treatment outcomes, such as my video records.