GPU miner here, been running dual PSUs for last decade. My typical rig consists of one motherboard, 6 or 8 cards, two PSUs 650-850W each, for total of 1100-1200W per rig. They are running for years on end, without any reboots.
I presume you just speaking from what you heard, not real experience?
I agree, that running two PSUs on one electrical device (like one Apollo or one GPU) is a no no. But don't extrapolate that to "One should NEVER try to run ANY sort of PSU's in parallel" (your quote), because that's BS.