Great find on that internal photo inside the FSP power supply! I'll be opening mine up when it arrives to look deeper inside.
So I did a little more digging and research (ok way too much research actually...)
I discovered there's another chassis option for the NUC9 Extreme compute element, which is the Cooler Master NC100. It comes with a case + PSU combo + slightly different baseboard, but still appears to use the same 10-pin connector on the baseboard as the Intel version. The PSU it uses is a Cooler Master V650 SFX Gold modular power supply. It uses a custom modular cable from the 10P + 18P connectors on the PSU side to the 10-pin BBWC1B connector on the baseboard. Reviewing some photos and videos, I was able to see which pins are present on the 10P + 18P side, and compared them to the pinout diagram of that PSU. Below is what I determined is going to the 10-pin BBWC1B connector. I have not been able to clearly figure out which pin is which on the 10-pin BBWC1B cable connector side yet.
Green highlight were pins found visually. Appears to be 13 total pins/wires on PSU side, with 3 of those being sense wires, which I assume those 3 are double wires into a single pin on the 10-pin BBWC1B connector. That leaves 10-pins for the connector on the baseboard side.
It seems the 10-pins should be 12v, 12v + sense, 3.3v, 3.3v + sense, GND, GND, GND, GND + sense, PS-ON, 5vsb.
So it's missing the Power Good/PW-OK signal?
Another helpful pinout diagram of the Cooler Master PSU, with pin numbers identified:
Helpful reference videos I used for the NC100 chassis/viewing the modular cable pins that were present:
Also, here are some pictures of the baseboards and 10-pin BBWC1B connectors in case these can help answer any questions:
https://www.kitguru.net/wp-content/uploads/2020/05/Baseboard-3.jpg
O1CN01cd7ibH1F81WVz7P2C_!!53-fleamarket.heic_468x468Q75.jpg (350×468) (alicdn.com) (backside of baseboard)
Hope this all is helpful and gets the discussion further along!