As he says, not yet developed or proven. And resistance could still develop to the phage.
Obviously, the phage would have to recognize the pathologic bacteria, enter it, and then induce some fatal change. From an immunological standpoint, the bacteria could become resistant by changing whatever surface marker the phage is programmed to recognize. Then the phage would be worthless, as it wouldn't be able to recognize the target.
Remember, the changes that provide immunity are not deliberate. They result from random mutation, not from intelligent design (got that, Orac?) and, if the phage could recognize more than one surface marker there would be less chance that the bacteria could survive. Considering the odds against even one surface marker changing by random mutation sufficient to provide resistance, by targeting multiple surface markers, the odds against resistance become astronomical.