most explanations or conclusions are based on wrong assumptions and misunderstood's
the main myth is: 6k source is not a downscale to 4k, 6k camera source has only around 3k true resolution and this only if the lens has enough overhead for this resolution and the right aperture was used like f-stop 5.6 and above and not more the 11... and the right framerate/shutterspeed
most lenses have a bad resolution at f-stop 2.0, 2.8, or even 4.0, but these are mostly used
also, the first video here has only HD-Resolution, not 4k
also, he does not use fixed scalings in his second's presentation and also the most viewers do not use HD-Monitors with fullscreen scaling with a pixel to pixel match to the original video resolution. So he adds another scaling... your monitor adds scalings... so you look to several times degraded 3k compared to 2k downscale, that's the simple reason why there is not such big difference between "6k" source and 2k downscale (of the true 3k source)... so most conclusions are completely wrong especially part 11 in the second video...