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.