Calibration is an easier concept for the masses.
Compensation is what is actually occurring.
The process of rotating the magnetometer in two axis allows for the characterization of the hard and soft iron distortions created by the components in and on the aircraft. Then an algorithm creates a polar compensation matrix correcting them.
I guess it’s easier to accuse folks of propagating BS than actually looking into it.
If you wish here’s just one article to read...
Hard and soft iron magnetic compensation explained
I hope you read this paragraph within your reference: "Earth ambient magnetic field as a function of zip code can be obtained from the online National Geophysical Data Center magnetic field calculator. But for some applications, you may not even care what the local value is. (That's why i really don't care when I recal the drone's compass 'fore flight.) The important point to take from the discussion above is that we are leveraging the symmetry of the data set to drive a solution. Compass orientation is determined by ratios of the 3 dimensions calculated. If those values are off from the expected values by a multiplicative constant, the ratios still hold."
Please note the noun, "constant!" Where these "multiplicative constants" occur gives rise to the maps showing magnetic declination with the familar isogonic lines.
OK, I don't know the answer to this question, but since you appear to be learned, I'll ask you, "Just how much is the drone's "compass" affected if the operator calibrates it where the mag dec is +6.0º and then flies it without recal where the mag dec is +7.0º? The difference in one degree of longitude at Greenwich is roughly 53 statute miles.
That of course, changes with a change in latitude. Greenwich is at 51.5ºN. That's 6 deg. in latitude further N than the latitude of Montreal, Canada. So, as you come S from 51.5º, the mileage for 0.5º in latitude increases until the Equator is reached. All of the continental US is at a latitude less than 49º30'N.
Please pardon the length of my discourse, but what I'm trying to show is just how different from true N a magnetic compass will indicate with position.