I see the information on the motor but not the bit about using the marker as a speed feedback, I do see the encoder sheet that shows the marker as home pulse, the only other thing I have seen the marker used for is a used as count verification of the pulses per rev, in case of missing encoder pulses,
If the marker was used as a tachometer, it would be virtually useless at low rpm.
I would hazard a guess that if there is reading heads above the inner track, that could be for digital tachometer, and it is possible the A & B channel are used only and although the disk has a marker pulse it may be just that they used a standard disk but it is not wired to take advantage of it.
Al