There’s a post that clearly says the 2 mini 6 pin to one 8 pin will work no issues but if you are going to over clock it you will need additional power and best thing is to do the pixlas mod.
This post that you read is completely wrong.
The 8-pin PCIe power connector draws up to 150W, so, you are powering it with a 6-pin mini-PCIe connector that is rated for 75W and can provide something around 110~120W before the SMC does an emergency shutdown of the PSU.
If you are doing nothing with the GPU, this can work, but the moment the GPU starts to draws more power you'll have a SMC emergency shutdown.
Nitro+ cards are heavily overclocked from factory and can draw 300W+ spikes/transients, this is asking for a defective backplane and not in the long run, but soon.
A Sapphire Pulse RX 580 that have ONLY one PCIe 8-pin power connector and is correctly connected with a dual mini-PCIe 6-pin to a 8-pin power cable, can make the SMC shutdown the PSU when running full throttle - with loads of reports here over the years.
Btw, even the Silent/Green/LowPower mode of a Sapphire Nitro+ draws more power than a Pulse.