I guarantee you that the controller box is grounded. 0 ohms between the black ground wire and the box. It is so by design as the mosfet that controls the heater current is heat sinked to the box, and that part of the mosfet is ground.
The controller box ground as the main ground is not adequate, as you would not want to ground the whole board via the mosfet heat sink, hence the separate wire. But grounded it is nonetheless.
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