My husband is legally blind because one eye is a prosthetic, but still can see well enough to drive, etc. So agreed, disabilities are not always apparent. That said, IMO, I think these folks were taking advantage of the service dog policy. I would have also complained to the kitchen staff about the dog being held near food. Regardless if the dog was not "doing it's business", it could have been shedding, etc.
My guess is that the owners use pee-pee pads for their dog. And in that case, I feel sorry for the cabin steward finding that in the room's garbage...