Great question and I have been thinking about this for quite a while, here are my thoughts:
This is a myth - until recently, most speaker reviewers did NOT measure their speakers so to correlate straight curve to boring is impossible if the reviewer does not have access to said measurements. Additionally, even if a speaker was flat in an anechoic chamber, in-room response could be completely different if the reviewer sat in a null where sound is sucked out at a critical frequency range. So even if the speaker was flat in an anechoic chamber, it's definitely NOT flat in the reviewers' improperly treated room. And most reviewers do NOT have an acoustic analysis of the room modes so they do not even know the location of these nulls in order to either move the speakers or move their chair. Worst case scenario, a truly flat speaker sounds boring, dull and lifeless because all these speakers will be subject to the same suck-out at all the wrong frequencies, and speakers that have peaks (like bright, non neutral speakers) suddenly sound great because they are actually "flat" depending on which reviewer's room they're in!
Listener fatigue likely? He wants to STOP hearing everything above 8 Khz because he spends his entire day listening/mixing in this fatiguing range so we should NOT be comparing ourselves to a professional who has listener fatigue at the end of the day. He wants to enjoy the music without the "sizzle" so he would likely prefer a roll-off on top to give his ears a rest, no?
Assuming your room allows your speaker to play in a neutral manner consistent with listener preferences as posited by the Harman curve, then after listening to your favorite music with speakers that are considered neutral/accurate, are you dissatisfied? Go to the core of your dissatisfaction and discern whether it's the source (the song was mastered for AM radio in the 70's and not digital playback in 2020), your room or your speakers. Sometimes what we believe is flat (speaker measurements in an anechoic chamber) is actually not flat at all and all you need is DSP like Dirac to make the speakers flat as they were intended.
I took the quote from the studio technician mostly because I suspected that it would trigger a discussion here.
People are complex and this with sound has a lot to do with psychology. That was what he said at the time. He's no acquaintance with me, so I do not know if he said that in order to provoke me. So maybe he only felt when he told me.
I may be far off now ( I may be completely wrong) so take it with a pinch of salt. Some different ideas and hypotheses:
He may be mixing and playing music that he does not like to listen to privately, but then it may be about Pavlovian conditioning, that is, sound signature X = music he does not like. If he hears music he likes with sound signature X, it is the speakers that are bad. Which leads to the fact that sound signature Y is what he prefers (even if it is colored) because he associates it with music he likes. Maybe a slightly far-fetched hypothesis, but you do not know.
He may be a Marxist: "Those are my principles, and if you do not like them ... well I have others."
His ears are tired and he wants a sound without details and sharpness when he listens at home.
At work, he sits in the perfect listening position, sweetspoot. At home, he listens in the apartment with kids running around yelling and playing, does not listen in sweetspoot but moves in the apartment when he listens. Then maybe four speakers, which color the sound a bit, placed in his room is preferable to two more colorless that are best suited in his listening room at home when the kids are not screaming and he can sit and enjoy music in the royal chair ,the designated listening chair (which does not exist in his home). Another hypothesis. I have no idea if that is the case. I'm just speculating now.
How do a couple of small studio monitors, with exceptional data measured on paper, such as Gelenec, do at his home IF he only listens to music at very low volume? This compared to a pair of speakers that have an extra boost, an extra increase in the bass range (on paper measured deviation from the ideal frequency curve, ie colored sound). A permanent loudness for those speakers, so to speak. Who knows, the ones in his listening room, with his furniture, at low volume might sound better (which also has to do with taste) than a pair of Gelenec.
Or is it some other simple explanation. He loves bass pumping music and has a couple of big a ... s subwofers at home. He is completely sold at high volume, home speakers with extreme SPL (loud is better).
....and so on and so forth....