So, I'll just add my own input here to start off the discussion, I guess...
I would say that so far, I think it's pretty relevant. I am often pretty confused as to whether I am an INFP or an ENFP, and Enneagram offers a pretty good explanation as to why that might be.
Point 1: "Tertiary Te" and "Integration toward 3": As an Enneagram 9, my "arrow of integration" is toward an Enneagram 3. In other words, if I am a healthy 9, I should start to look more like a healthy Enneagram 3 (whose basic desire is to be seen as successful, to be admired by others). The Enneagram 3, in my opinion, has a correlation with Te. Because as I get more healthy, less slothful (stereotype of the 9), I should also become more active, get-things-done, achieve things so that I can be successful, or be seen as successful (3). These are right in the wheelhouse of Te, which helps me to organize things, get crap done, etc.
Point 2: "Inferior Si" and "Disintegration toward 6": However, in the same way, when an Enneagram 9 is under stress, they start to look like unhealthy 6's (who desire stability, security and support). Extremely anxious and worried, very cautious and indecisive. This, to me, correlates with inferior Si. When I am under stress, I overthink things in my head, think that I might be missing some sort of detail (and that one little detail would just mess with everything). I get very worried and concerned about my health. I look back at my past and start to think that I made critical errors in the past that will compromise my future security/stability/happiness/etc.
Point 3: "Ne+Fi" and "Basic desire of 9": However, the primary desire of a 9 is to have "peace of mind," peace with their external environment, harmony with others. This is essentially, in my mind, either the function of Fi (inner peace that you are living by your values) or Fe (maintaining harmony with others). As a result, I was confused for a while as to why it appeared like I had Fe. This, in reality, was just my 9-ish tendency. And my Ne + Fi was working in conjunction with each other to achieve this desire to have external harmony. So, Ne would pick up on the the general pattern of facial expressions, body language, and Fi would interpret this information (what kind of person this is, what they like, what they don't like, etc.) so that I would be able to mold my approach to the person and maintain harmony with them.
But, the thing is, the Enneagram 9 is not very closely tied to Ne as much as, say, an Enneagram 7. In other words, even though I have this toolbox of Ne + Fi + Te + Si, my Ne doesn't get as much exercise as it would if I were an Enneagram 7 ENFP.... because Ne is mostly used in conjunction with Fi to explore my desire for harmony with myself and with others. It isn't necessarily always used just for its own sake, for the sake of exploration (having new experiences) because my basic desire doesn't call for it.
So my top half aligns more closely with an INFP (Fi gets more exercise than Ne). But my bottom half aligns more with an ENFP (Te/3 as integration/development and Si/6 as disintegration/stress behavior). This to me perfectly explains my own constant dilemma as to why I seem/feel like an ambivert, and why in some ways/times, I feel more like an ENFP, and at other times/ways, I feel like an INFP.
Well, the end. Maybe MBTI and Enneagram should not really be used in conjunction like this. But since I found it helpful, I thought it might be helpful to you guys. And I would like to know what you think.
And of course, if you think that I am wrong or that I am clearly either an ENFP or an INFP, feel free to critique my theory. (Lol, I wrote this basically stream-of-consciousness, so sorry if it seems a bit incoherent.)