In order to be cognitively ambiverted you'd have to have an equal grasp of both the dom/aux function, and use them interchangeably. I just don't think that happens.
But I think absolutely in terms of the four dichotomies I'm a real ambivert. It was actually the Keirsey test where I was perfectly 50/50 on E/I. Also when I took the official MBTI it spit out INFP then changed it to ENFP after I answered a few more questions.
I think the theories may overlap a bit, though. Even in Keirsey's Temperaments I felt like an almost equal mixture between NF and SP and neither one perfectly, and was only one point difference between NF and SP on the Temperament test in the back of the book (which is different than the four dichotomy inventory). Surprise suprise that I also mixed up Ne and Se in function theory.
This is something else, though I'm getting into.
I think healthy people are actually closer to ambivert, anyway, which is what I believe wildcat was saying. People who are extreme extroverts or extreme introverts aren't very balanced, and are possibly in dom/tert loops...or they're just young and haven't developed their opposite functions as much.