it's not necessarily a bad idea what he's doing in the video, but it's clearly not accounting for some ear canal resonances. depending on how he places the mic the 3khz might be wrong, and something around 10khz also is boosted in the ear, and I doubt it would show that way.
also of course the mic used would need to be calibrated.
but in the end his idea of flat follows electrical flat. it's not a bad compensation at all and I'd be fine if it was accepted as audio standard for headphones like it is for amps or DACs already. but an electrically flat headphone does not sound neutral IMO.
look at Alchi's graphs for the stax, the measure at the bottom is RAW so when a headphone measure is a flat line there, then it's electrically flat(again with a mic that is good and compensated). you see how the 3khz rise on the stax that Alchi calls neutral. all "neutral" sounding headphones have a bump in the 2.5 to 3.5khz area. this has to do with our anatomy and an electrically flat 3khz isn't bad, but does sound a little recessed.
if you're curious about this I suggest you look at this vid
lot of good information in it regarding the ear and the limits of measurements IMO.
aren't the top lines the diffuse field compensation applied? if that was flat would you feel like it sounds neutral? I tried and I don't.
my perceived neutral on headphone seems to be somewhere between diffuse field and the harman target, so I would most likely find this stax to sound rather neutral(never tried).
for vented IEMs I also like about the same kind of FR. and for sealed IEMs I seem to prefer more bass and more rolled off trebles for some reason. but in the end I can't say if my perception of neutral would fit other people so I join you on this, "it's best to experiment and discover what is most enjoyable to you"