That's not something that needs to be measured externally.
Also, one wonders just how horrible their production tolerances must be if something like this even needs that sort of calibration in the first place. No other company does this. Nonetheless, even if one accepts this bizarre excuse at face value, it doesn't take a genius to realise that the firmware can "self-calibrate" trivially: If the EEPROM in the sensor is uninitialised, then it just needs to record the max/min value the first time it's closed, and save that as the "lid closed" point.
The only plausible explanation for doing all this extra work is that this isn't merely incompetence; it's intentional malice. Given how much Apple spends on lobbying and other hostile activities, this is not surprising.
What do you think calibration of digital devices entails?
It does involve exactly that. Whereas in an analog device you would be adjusting a potentiometer or something similar