AFAIK oratory has access to a measuring rig and people send him their headphones to have their specific unit measured. He creates EQ presets manually based on these measurements to equalize them to Harman and shares it with the community. People who like Harman use it, otherwise EQ by ear or don't EQ at all; it's entirely preference.

The goal of the Harman curve was to define what the average listener would prefer. A headphone that matches Harman should sound and measure like a pair of flat speakers in a studio. The bass on that target is entirely subjective and something they've adjusted through revisions. Love it or hate it, it's important to have reference curves like Harman/Free field/Diffuse field so that we have a point of comparison between different sets.

Every measurement has context, yes. It's why graphs made with the newer B&K rig are not 1:1 compatible with older GRAS rigs, causing people to rebuild their measurement database. I don't think it's the margin of error that is the issue, the rigs are plenty precise. Accuracy is what is questionable; how certain are we that a measurement represents sound in the real world? Even if you control placement and seal, there will still be unit variance and differences in physiology. Even then FR graphs are mostly reliable for telling you the overall tonality of something.

I'm not an expert and may be wrong but from what I understand:

1. Measurements are an imperfect representation of personalized sound. Unless your ears and head match the rig exactly, what you hear is not what is measured on the rig. Placement of the headphones, pad seal, and unit variation will change the FR at your ears. Treble measurements are also inaccurate on the older standard rigs, above like 10kHz or even lower the measurement is unreliable.

2. The harman curve, as well as most measurements, are extremely smoothed and zoomed out to make them easier to read, but you lose the nuances in return. All the little dips and peaks that you don't see on the FR change the sound. It's basically impossible for two headphones to have the exact same raw measurement and thus sound the same, even if their smoothed compensated graph closely matches.

EQing to harman gets you close to the overall shape, but the small dips and peaks will still remain. It's like trying to draw a perfect circle by hand. You probably won't succeed and there will be different squigglys each time, but people will still recognize it as a circle.