He is using the 2 x PCIe 8 pic cable to 16 pin HPWR cable in reverse (which seems to be intended). The 16 pin side of the splitter cable plugged into the PSU, and the 2x8 pins plugged into the graphics card. Typically if your graphics card needs 2x8 pin PCIe cables, then you plug in two separate PCIe 8 pin cables to and from the PSU.
Looking at the US website for your particular PSU, it shows that the following cables come with the PSU:
Power Cord x 1
Motherboard Power Cable x 1 (450mm)
CPU Cable x 2 (550mm)
PCI-E Gen 5.1 16-pin Cable x 1 (450mm)
PCI-E (16-pin to 8-pin-8-pin) Cable x 1 (450mm)
PCI-E 1-to-1 Cable x 1 (450mm)
SATA 1-to-3 Cable x 2 (300+200+100mm)
Peripheral 1-to-4 Cable x 1 (300+100+100+100mm)
Addressable RGB Cable x 1 (800mm)
User Manual x 1
ATX to SFX adapter bracket x 1
Considering the language above, I would expect that the 16 pin to 2x8 pin cable you are using is being used as intended by ASUS. I think you can strike this off list of potential issues. Typically when a PSU is not working properly it will shut down the PC, not cause any sort of performance issues like stuttering or frame drops.