It is but not through the DC “ground” trough the power cables but through the motherboard via the case and via pin 18 and 19 on the main ATX which should lead t

It is but not through the DC “ground” trough the power cables but through the motherboard via the case and via pin 18 and 19 on the main ATX which should lead to earth but not always. If the motherboard is not connected via metal stems and screws through the holes with an earth ground plane it won’t be grounded this is why there are either solder dots or a copper ring around the screwholes in the motherboard and why you should not use plastic standoffs unless they are explicitly specified and then often if you use metal standoffs you might screw something up. For mining you essentially connect cards via PCIe raisers which often use a USB 3.0 cable and the card is connected to 12v rails which may or may now lead to earth and often they won’t. It’s really not that hard to screw it up.