If you're relying on Mach for threading, then you don't need closed loop. Mach only needs, and can only handle a single pulse per revolution, which it then uses to control the Z-axis, so the spindle itself doesn't have to run closed loop. The spindle just has to maintain a relatively constant speed, to ensure things remain in sync.

If you are going to be using some form of external motion controller, then the spindle and feedback requirements vary.