Amir sees the Harman target as valid because of A: the extensive research done and B: it matches his personal observations.
Especially the latter, given the always subjective evaluation, validates the research for Amir and most likely also for Oratory.
It's a choice that has been made and IMO makes more sense than diffuse field correction for speakers.
Until there is another standard more suitable for headphones on 1 particular test fixture which can be created for other fixtures as well the Harman one makes sense. Those that want measurements without the 'preference' part for the lows can just imagine a 'horizontal target' below 500Hz.
I still get the impression that his subjective observations pre- and post-EQ are heavily biased by what was measured. I'm not saying that he's lying about his subjective evaluation. I think his bias makes him judge headphones that deviate from the Harman target too much,
to his eyes, as sounding bad to his ears.
He knows very well what a headphone that adheres to the target should sound like. And if it doesn't sound like that, by default it's bad, because he doesn't hear what he wants to hear (something resembling the Harman target curve).
I'm not saying that he's not allowed to have his own subjective target that he likes. He like the Harman target curve and I'm perfectly fine with that. I'm less OK with the way he represents it as a hard fact that any deviation from is objectively bad/wrong. The line is drawn too hard.
It's much easier to objectively judge a headphone on THD.
In contrast to amp and DAC measurements we can say that we know what we measure. We want 0 THD, 0 noise, 0 deviation from a flat frequency response, 0 IMD, ... Because objectively that is perfection. Subjectively some may not like that. But it's very clear that that is an subjective deviation from an objective optimum.
I'm not 100% convinced that the HTC is such an objective optimum. It's a subjective target that many like, because it represents something that comes close to good sound from speakers. It is not objectively the same as the sound from those speakers.