I think you are referring to how the physical earth ground in a regular electrical service is connected at the main panel with the neutral, and how essentially grounding the motor casing gives it the same potential as the neutral(hopefully assuming the ground is electriclly clean) and would provide protection from shock in the case of instead of electrifying the outer case due to a failure, the breaker would trip disconnecting power to the circuit
however in this case it seems this motor isn't going to be connected to the mains, and so it wouldn't protect it from say the 12v supply shorting to the case, not that that would hurt anyone anyway
All your questions could be answered if you were to find the datasheet