It seems to go that:
a.) We cannot agree on many things about MBTI, therefore the theory cannot be applied.
b.)If we all agree "for the sake of argument" that the theory is to be applied, then it still internally has space for wide interpretations.
interpretation is equally valid/invalid (in effect).
d.) So back to point A.
There is a subtle leap from b.) to c.), but it's a big one, and I am suspicious of it.
I would think that just because JCF has been applied in many different ways, does not mean there are no common understandings of it, nor that some are not much more widely accepted in the typology community than others.
but there is still enough of a continuity and common "thread" there in order to make the idea of a coherent hypothesis and debate conceivable.