We have internally used a blended metric similar to the Gm Gedlee metric for quite a few years. I know at least 2 of our competitors do as well. I agree on his basic premise that mechanical systems (speaker drivers) do not have as much higher order distortion, but on a practical basis, with multiple drivers, how distortion presents is more complex. One missing element is frequency weighting.
In one of the links, 2nd order distortion was described as euphonic. That may be true at a high enough level. Using the Gedlee metric, at the levels indicated, it would inaudible or close enough. Almost any DAC today would have a Gm close to 0. Not just expensive ones, all of them.
The vinyl vs CD article talks a lot about vinyl, not much about CD. One small, near useless section. I lost confidence in the author over a few items. Changing the load on an MC cartridge (135 ohms), from100pF to 200pF reduced the 3rd order distortion? I have done enough DIY electronics to know that smelled funny. That would change an RC filter from 12MHz to 6MHz. I say bad amp, bad switch setting, or multiple plays. Raises an issue with phase in filters on CD while ignoring that vinyl likely has phase shift too. Show digital samples that look bad, say it needs a reconstruction filter, then don’t show one? Bio attached to the article. Really impressive! He must be an expert? BS-Zoology, BS-Psychology, MS-Physiological Psychology, Ph.D.-Neuroscience. Welcome to 2022, everyone is an expert.