This seems to be exactly how I operate as well. Right on! I let people see the tiniest nook of the door to my core but 98% of people never step through it.Oh. Yes.
It should be said however, individual (and even general "type") values vary largely, which will account for the discrepancies in various perspectives on the paradox of INFJ openness/closedness. For example, I've been surprisingly okay with revealing horrific family events with friendly acquaintances when/if the topic were to surface and personal experience would serve the discussion appropriately. (Topic examples that I have quietly shared with a small group of coworkers during a late night pow-wow: My uncle killed himself in our garage when I was 6 years old. My grandfather murdered my grandmother (and my namesake). I ran away from home at 14 and didn't return for 7 years.) All of which is true information that would be revealed MUCH to the horror of my SJ mother if she knew. It isn't that they are "light" topics to me --just that I don't feel they are significantly revelatory about me as an individual, therefore I feel comfortable sharing these facts if they serve my point or goal.
Contrarily, my mother's incessant desire to impress her friends with things like "my daughter wants to teach" and "she's been writing since she was a kid" makes me cringe. For reasons completely incomprehensible to her, I do not want this information shared with just anyone because it reveals my passions, therefore it reveals me --whereas my family debacles feel less personal as they are simply "events" that have shaped the course of my serpentine life.
There are also people who have known me since birth that do not know huge swaths of my life. For example: they would not know that I have survived several serious bouts of suicidal depression, was baptized on my own at 14, decided I was agnostic at 21, was seriously abused for many years by my own mother, or even my favorite song etc --yet I share this information freely with a forum of complete strangers. My choice of self-disclosure is fickle, circumstantially dependent, and purpose-driven whether the listener knows it or not.
For every inch I reveal there remains a mile underground. Ultimately, INFJs spin their own intricate web of values that seems complicated to many but the makers.