Xavier NX 40-Pin Header Backpower

I’ve created a custom PCB that interfaces with the Jetson Nano. My PCB provides power to the Jetson Nano via the expansion header (not through the DC jack). This works just fine on the Nano and I expect this should also work on the Xavier NX however there is a circuit of concern:

The description is a bit confusing and seems to contradict the actual function of the circuit, which is to prevent backpower of the devkit from an expansion board. I find its inclusion strange as it would make more sense if the circuit were mirrored such that VDD_5V_SYS and VDD_5V_GPIO swapped places - this would allow bidirectional power delivery for positive voltages, but if the expansion board supplied an incorrect polarity, the developer kit would be protected from reverse polarity.

As a test I modified my carrier board to add a short between Q23:D and Q23:S which I thought may allow the expansion board to power the developer kit, however the NX still doesn’t power up from the 5V provided - I assume there is an enable signal somewhere that prevents this from working.

I’m aware of the PoE backpower header (J18) but my expansion board is only designed to supply 5V, so this is unsuitable.

Is there something I have misunderstood here or is this a mistake in the design of the carrier board? Is there any further modification that can be made to the NX carrier board to achieve the desired outcome?

The back power design is on P3509 only. If you are using 5V only for the NX carrier design, you can only follow reference power part design of P3449 to take that power on/off logic design. All that are for auto power on function which is to enable POWER_EN signal when power plug in.