Careful, that link actually doesn't apply to the Duet3.
It works for my Duet3 6HC without major issues. The only piece of information missing is to pull the SBC jumpers.
Yeah you're right. Every time I look at the schematic, I get a headache.
The description for the PS_ON header says...
3-pin KK labelled PS_ON: open drain mosfet output for controlling an ATX-style power supply or a SSR. The +5V pin can also be used to provide external 5V power. A small amount of 5V power can be drawn from this pin (through an internal 220 ohm resistor), so that the control terminals of an SSR can be connected directly between the +5V and PS_ON pins. Note: on the v0.5 board this connector is rotated 180 degrees compared to the intended orientation on later version boards.
It doesn't actually say that you can PROVIDE power via that header.
The schematic does show that you can input 5V from that header but if you try to draw 5V from it, it goes through the 220ohm resistor.
The power options do seem to be spread out.
https://duet3d.dozuki.com/Wiki/Duet_3_Mainboard_6HC_Hardware_Overview#Section_Power_distribution doesn't even discuss the PS_ON header.