I also don't think he is playing out of position. He is a one on one matchup nightmare for most players regardless of position and is a plus defender overall against 2s and 3s.
I will say though, he needs to get better court vision, as he is all to often missing teammates who are wide open, especially when he brings the ball upcourt. He is constantly missing guys wide open ahead of him for easy runs at the basket when he brings the ball up.
And his handle....he has to get like Brown and bend more, keep his dribble down and keep it tight to his body. Tatum has a remarkably high dribble that is really loose. Gotta make that better.
Oh, and lastly, learn to finish at the rim with the left had, Jayson.
He does all this, and he will be an MVP, regardless of what position he plays.