Or at least those are the reasons they state.You wouldn't want it to be, due to the SLD effect: acoustic sources at different distances from the ear are perceived to sound differently even if the measured response is the same. See Theile's seminal paper. Then there's the lack of tactile bass from headphones (even less from IEMs) compared to speakers which needs to be compensated for. These are both reasons why preference adjustments are needed to arrive at the target, just as Harman did.
That excerpt keeps talking about the problem of free field measurements and how the diffuse field is a better starting point for devising a headphone curve. All from 1986 no less. No one is arguing for or against any of that that here. I thought we had moved well beyond this at this point. Anyway, you can have your Harman. I just don’t care and regret getting involved.