I believe we should differrentiate between skidding/slipping and yawing.
We use rudder in a turn to YAW the a/c in order to prevent it from skidding or slipping, i/e to keep it in balance. That has nothing to do with the fact that the a/c is actually yawing constantly in a turn and i believe that is proved by the turn indicator.
Such instrument is ment to detect yaw, with a gyro mounted with its axis parallel to the longitudinal axis of the plane.
It would not react to any roll or pitch, only to yaw.
Anytime it is telling us we are turning, what actually means is that it is sensing yaw... or not?