It's possible he might be hearing IMD on the headphones way below 21 kHz, isn't it? "True" hearing around 21 kHz at 26 would be quite unusual, but not impossible.
Also, it seems somewhat pointless to claim you can hear differences between DACs with different reconstruction filters. Sure, I can use the super slow filter with severe roll-off past 10 kHz and ABX it with the standard filter (doesn't have to be another DAC) and I might be able to pass the test. Doesn't mean I could ever differentiate between different DACs with identical filters. The main point of the video "DACs matter" is thereby wrong, as it's more something like "reconstruction filters matter and specifically these two I tested only really matter for me as I can hear higher tones than 99.6% of all adults".
Meh.
This is one of the few remaining possible spuria here, but I'd propose that it's
more fantastical to imagine that Cameron can hear the distortion products (themselves =/<-40dB from signal level with a Susvara in that frequency range) of a very small band of high frequencies which are =/<-40dB versus his signal level. It's not
impossible, and it's something we'll be looking at in future ABXs, but such low magnitude nonlinearities would almost certainly be masked by even quiet audio band signals.
Like, comparatively, "an individual can perceive slightly above 20khz" seems a lot less magical hearing to me
Of course, nothing in the video says that this is a major or even significant difference, and it should be completely obviated simply by not using 44.1khz sampling with the rare tracks that actually have 20khz+ content.
Also I was waiting he would mention intersample overs and if any attempt was made to control them but no, so unless I missed it there’s reasonable doubt that the differences he heard are intersample overs which are likely with the steep filter he used.
A good question! This is, indeed, why the output waveforms from the DAC were recorded - if you see any apparent intersample peaks there, please point them out, I'd love to have another explanation for the outcome of this test.