Read the quote I posted above from Dr Sean Olive.
If you go back to that thread (from which your quote came) - it is clear that Dr Olive was referring to work identifying PERCEPTION.
ie: most people's PERCEPTION of Neutral (which as we know, is impacted by sound levels and by loudness curves, etc...) - aligns with the Harman curve (as it was developed based on an average of the tested peoples perceptions!!
True Measurable "Neutrality" - is a whole different beast - and this is particularly indicated by Dr Olive's specific reference for a different curve when used for Binaural and/or HRTF purposes (where, I would suggest, true "measurable" neutrality" would be a requirement).
Also when considering that this therefore becomes a discussion about perceived neutrality, focusing on stereo recordings, listened to through headphones (without HRTF application...) - the volume level at which the listener chooses to use the headphones will in turn impact the listeners perception of neutrality as per the standard loudness perception curves...
Which of course implies, that different headphones will sound best at different volume levels - and that different users preferences, in terms of headphones, may also be driven by the levels at which they listen.
If you combine loudness curves, with a measurably flat/neutral headphone response - and set the listening level to the average used for the Harman tests that defined the Harman Curve.... do you end up with roughly the Harman Curve?
If not - then there is something else going on as well... if so then we have an understanding of what drives preferences, in a consistent system.