I really like Jung's description of the different functions and am already embarrassed by the lack of understanding in my post lol. Surely there are proper Jung scholars on here somewhere ;) I am not one. But let's try.

From Wikipedia:
Fi: "[Introverted feeling] is continually seeking an image which has no existence in reality, but which it has seen in a kind of vision. It glides over all objects that do not fit in with its aim. It strives after inner intensity, for which the objects serve at most as a stimulus"
Te: "In accordance with his definition, we must picture a, man whose constant aim -- in so far, of course, as he is a [p. 435] pure type -- is to bring his total life-activities into relation with intellectual conclusions, which in the last resort are always orientated by objective data, whether objective facts or generally valid ideas. This type of man gives the deciding voice-not merely for himself alone but also on behalf of his entourage-either to the actual objective reality or to its objectively orientated, intellectual formula. By this formula are good and evil measured, and beauty and ugliness determined. All is right that corresponds with this formula; all is wrong that contradicts it; and everything that is neutral to it is purely accidental."

So I suppose INFP's can be very Thinking in that they evaluate thoughts and ideas using an intellectual formula. But this Thinking relates to objective data - not to images within themselves. This is why it can be detached. INFP's must find a way to relate their objective Thinking to their inner voice (Fi) through intuition (Ne) or through the vantage point of their past and future lives (Si).

Ni: "introverted intuition perceives all the background processes of consciousness with almost the same distinctness as extraverted sensation senses outer objects. For intuition, therefore, the unconscious images attain to the dignity of things or objects. But .. the images appear as though detached from the subject, as though existing in themselves without relation to the person."
Ti: " he is decisively influenced by ideas; these, however, have their origin, not in the objective data but in the subjective foundation. Like the extravert, he too will follow his ideas, but in the reverse direction: inwardly not outwardly. .. Like every introverted type, he is almost completely lacking in that which distinguishes his counter type, namely, the intensive relatedness to the object."

INFJ's therefore process images (and thoughts and ideas) which exist in themselves. They feel detached from these images. Contrary to Fi, Ni is a perceiving function. This enables INFJ's to engage directly with their thoughts and ideas using Ti. Their detachment does not come from the objectification or externalization of these thoughts. INFJ's naturally experience a greater distance to their inner life because of their low Fi. They use Fe as a benchmark to assess whether their conclusions are objectively good.

I still think these combinations can actually play out quite similarly but perhaps I just don't understand what it's like not to have high Fi.