He can draw the line wherever he wants, however we should also be aware that he does not have any zilch of an authority to do so for others (this has been a traditional problem inside major religions, the discrediting of variations of the same religion, which can take the form of excomunnication and so on). If they call themselves christians and if they seem to be honest about their intentions, then they are so, period. That's almost like a brute fact. If they are following Christ in a completely different manner than other people, then that's just another fact.
To me, that's somewhat besides the point. What matters is that people become aware of what happens when you start taking woo too seriously and disregard actual scientific medicine: you let your own children die and then lose the rights to be parents of your other children.