Hmm, that's interesting, because I had never thought of the introvert/extrovert dichotomy in quite that way. I don't know what the psychological definition of introvert is, though, and maybe my mental definition is a little off. Sheppard on the show seems to be a man who prefers his own company to that of other people -- he's occasionally seen off by himself reading a book, or wandering around not really interacting with people and/or fading into the background in a social situation -- which is, to me, the very definition of introversion. When I read the ESTP description that you linked to, the showmanish part of it definitely does not seem like part of his psychological makeup to me -- his behavior on the show does not seem like someone who wants to be the center of attention, but rather, someone who doesn't give a rat's ass what other people think or whether other people are around at all.

I know people who are very inwardly directed and know nothing about their own motivations at all (at least, nothing accurate -- an introvert can spend all the time in the world analyzing themselves, but if they're basing it on flawed assumptions -- e.g. "the world is out to get me" they're not going to obtain good conclusions).