This description of the stable points for half and full step are with respect the holding torque and applied load. This holding torque is significantly lower when microstepping, however you can still move the stepper to a microstepped position. Microstepping is not just for smooth movement, but you do have to have sufficient torque for the load. If you do have insufficient torque to hold a microstepped position, then the stepper will slip to a half or full step position where higher holding torque can maintain the position. This means you need a stepper with higher torque for that application.
The only time I've read material on microstepping error is with respect to the actual held angle. The higher the microstep fraction, the higher the error (steps are not equidistant).