Help with Asus ROG GA15DH non-boot condition

Guy brought this machine to me for help. It's a BRAND-NEW (but mfg date was 2020), unboxed only twice (I unboxed it the second time), GA15DH-AH772. Don't know why the family never used this machine, but it's been packed away for several years now.

Specs: Ryzen 7 3800X, 16G DRR4, GeForce RTX2070S 8GB, 512GB M.2 SSD, 1 TB SATA

So, he brought the machine to me in the original box and packaging saying it wouldn't power up. There's a single orange light placed right next to the graphics card that lights up when the PSU is switched on from it's backplate switch (i.e., not the case switch), but no other components power up.

I tested the PSU with a multimeter while it was attached to the mainboard via the graphics card power connectors I got nothing, so I assumed PSU failure. Problem is, when I pull the mainboard power connector from the board, short pins 16 and 17 on the connector and power on the PSU, I see the CPU, graphics card and 1 TB drive powering on. Tested again the same way but with graphics power disconnected and I get the correct voltages from those connectors.

The PSU is a Great Wall NZXT E500 80 Plus Gold 500W modular PSU. I've tested the machine with a known good GPU - same result.

I'm thinking something's wrong with the mainboard itself - but looking for confirmation from experts. I'm kind of at a loss as to what to try next. I can't find a decent motherboard manual at the ROG site for this machine, so I'm not sure if I'm testing the power switch pins correctly, but I tried that as well. I tried shorting them to see of maybe the power switch was somehow at fault.