I'll have to read that section, although the graph relates to pure tones -- more broadband signals like a lot of music are not perceived nor behave the same way as pure tones. I think there is some confusion here in regards to the micing technique, whether it's on my side or the two of you, I'll have to read up more on this issue.
In any case, this affect would also be present at the mixing/mastering position and should be fixed there. I don't know why you say it cannot be easily addressed during mixing or mastering, where far field listening should always be part of the process. I'll further note that speakers employing a "BBC dip" do not, in fact, measure as a flat slope in-room, they show a dip. Speakers that are flat through the upper midrange/lower treble and have good directivity DO measure as a flat slope in-room. People should stop mixing/mastering on speakers employing a BBC dip to avoid that region sounding elevated on more neutral systems -- hence we circle back around (
) to my Circle of Confusion comment.