I think Eminem himself is INTJ, so I sort of see him as INTJ in the movie too. But, you might be right about ISTP in the movie.
I love the scene at the very end after he won the battle and they're out in the street. His buddy asks him if he wants to work at the club with him and host the battles, make a name for himself, and all the guys are saying let's go out and celebrate tonight, and he just looks at his buddy and says, "Thanks, but no thanks. I think I just have to do my own thing, if you know what I mean." And he walks off by himself, leaving his friends behind.
That scene really strikes a chord within in me in regards to my experience as an introvert and human relations. There's a desire to connect with friends and such, but sometimes............................you just have to do your own thing. It's sometimes hard to explain, but sometimes you just know you have to go off on your own and take a different path.