I think that just proves that the underlying structure of functions exists even within the Myers-Briggs theory. Myers does not ignore the functions in Gifts Differing, but in fact, organizes the types by them, i.e. the introverted thinkers (INTP, ISTP), the introverted feelers (INFP, ISFP), etc. I think you would agree that IxFPs are very different from ExFJs.
So when I say Fi in place of IxFP, or vice versa, they are synonymous, at least in my mind.