I didn't know Kiersey used MBTI, I thought he had invented his own system of 8 types. Why would he use the MBTI?
But even Kiersey's NF type description fits Arnie better, whatever his I/E. Don't try to read Arnie's mind, just listen to the things he says. (There's no evidence that he masks his introversion.) He oozes F-ness. No, he's not going to put a pretty flower or butterfly on his Facebook profile pic. That's just not the kind of evidence one should consider meaningful.