Well, no, you're both wrong. 6 pin PCIE power cables are rated for 75 watts. 8 pin PCIE power cables are rated for 150 watts!
Sometimes, we should listen to what our hardware is trying to tell us.
It looks like it has 17 amps (204 watts) available for the GPU. Rather disappointed that they didn't use a 6+2 cable for this PSU. The PSU should supply you with the power that you need to run your system, (at least what you've told us of it) (hint, hint, fill out your system specs) but Seasonic may have limited your ability to do so with the connectors provided. W1zzard tested a RX-570 4GB and it maxed out at 180 watts. If you subtract the 75 watts that the card can pull through the PCI-e slot, that still leaves 105 watts to come through that 75 watt rated 6 pin. I have no idea if Seasonic has over current protection on those connectors.
Possible solution: Get a 4 Pin Molex to 6 Pin PCI-Express PCIE and a dual 6 pin to 8 pin PCIE so you can pull some from the molex connector as well.