Isn't Turner not really thought of for defense?
No, but they do not need him for his defense. Really, all they need for him is to be better than Orlando Johnson on offense. Indiana is by far the best defense in the NBA, but they are the 17th ranked offense.
I cannot stress enough how much better Indiana is on defense than the other teams in the league. Indiana gives up 3.9 points per 100 possessions less than the 2nd ranked defense. To give you an idea of how big of a gap that is, 3.9 points is what separates the 2nd ranked defense from the 9th ranked defense. The difference from the league average defense and the worst defense in the league is exactly 3.9.