I think this may be the statement that caused the confusion:
(Found
here)
But it doesn't actually mention the gain modes. It mentions lower quality audio, lower quality headsets and a "low power mode". I think maybe people took "low power mode" to mean the normal gain setting, but I don't see any reason to assume that. I suspect that he might actually mean that setting the Hi-Fi DAC to "Off" in the sound settings actually puts it into a low power mode, bypassing 3 of the sub-DACs while allowing the phone to still utilize it for lower quality audio. That's speculation, since we don't have full details of exactly how the DAC works, but I don't think there's anything in the statement suggesting that the Quad DAC isn't fully active once you do turn on the Hi-Fi mode, regardless of gain setting