I would disagree that's the only difference. The main difference for me is in the knee chamber position. For a back kick, the chamber is underneath the hips, knee pointing to the ground. For a turning side kick, it's chambered at atleast 90° to the standing leg (ideally more), out to the side. Of course the arm position and the amount of turn in the upper body (shoulders in a line to the target for the turning side kick and back almost completely facing the opponent for a back kick) are important too, but it's the chamber position that is for me the key difference.
I didn't say it was the only difference...
I'd say the chamber is also a major difference, but if you don't turn far enough, the chamber won't matter. Because you're not going to hit the target anyway. Which is more important? Flip a coin.