Understand all that, but with my knowledge of electrician diploma, pre cursor to full fletched, the case itself is not grounded. It are the ground wires connected to the circuits which in turn are grounded via either a copper pole or other form of grounding a house.
how the ground of the psu itself is externally connected to a case… well, I wouldn’t know how. I’ve built PC’s, never with a external grounding connector coming out of the psu.