It wasn't clear to me from this that these dogs had any special cognitive abilities, as opposed to neurosensory abilities, e.g. maybe most dogs can't audibly distinguish human vowels and consonants well enough for this to work.
I wonder this as well. In my experience, dogs have a MUCH easier time learning hand signals than learning verbal cues. I wonder if dogs would generally be capable of learning many more words if they were taught in sign language.