If it has 12 volts AT the motor, the motor should turn. If the motor were burned up, it would draw enough current to blow the fuse.

If there is a open wire leading to a winding in the motor, or completely worn out brushes then you could have voltage there without the motor turning.

Will the motor turn by hand? If it does, then you likely have worn brushes in the motor. If it doesn't turn by hand and still shows 12 volts that would be strange.

For sake of describing volts and amps, compare these to pressure and volume of water through a hose. The voltage is much like the pressure applied to the hose. If there is no restriction at the point where you have your pressure gauge, then the pressure (voltage) will drop because there is no resistance to maintain pressure, but the volume (amps) will be very high because the water can flow freely. If, however, you have a very small hose attached where your gauge is placed, you will have lots of pressure (volts) because the water can't go anywhere, but you will have little or no volume (amps.)

So, using this analogy, you have 12 volts at the motor. That means that the controller and associated wiring are providing a circuit to the motor. Your meter is completing this circuit, but with a very high resistance to current flow, so voltage is maintained there. If you had a short at the motor, the 12 volts would drop and would allow enough current to blow the fuse. This indicates that there is an open between the point where you are measuring motor voltage and ground. That open is likely to be brushes.

Your uncle is partially correct. The circuit is supplying 12 volts at the motor, but might not be able to supply enough current (amps.) If, however, it could not supply enough current, then this would mean that there is a high resistance upstream and you would not have 12 volts. That is, if the motor had normal resistance and the circuit could not supply enough current, you would not have 12 volts at that point.

I hope this makes at least a little sense.

Good luck,