i think the enneagram adds interesting points to this. especially when you describe the enneagram types as ego fixation. it seems to me there is a very distinct difference between 4w5 and 9w1 enneagram infp types. most of the 9s i know seem wayyyy more open than the 4s.
i agree totally that it is a way of being guarded. i notice a huge difference between infjs who are 5s vs infjs who are 4s or whatever else. 5s are paranoid. they use cynicism to round out the flavor of their world. it's like vanilla bean, really ties the room together. keep the world at a distance (Ti!), detach, and prevent your experiences from being routed thru the lens of the Self that hurts so much when it (the world) and your action, desire, dreams, etc are REFLECTION of you (rself) in it.
i also feel like there's a really big difference based on introversion/extroversion balance within the continuum of a type. how some infjs can be so Ni Ti and very unexpressed via Fe Se. or infps that are more Fi Si and get trapped in endless remembering, reliving the vivid emotional past (seems very enneagram 4 to me), whereas others express more and practice using Ne Te quite a bit more (and are probably 9s). get out of the traps of negative feeling, and make new, new, new.