OP said "the instant the quad starts moving, the compass heading is absolutely and perfectly established".
Yes, true. The known COURSE of the vehicle is now known at this point, but is it moving forwards, sideways, backwards relative to the body of the quad? The compass is needed to determine the orientation of the quad relative to its course, otherwise flight controller corrections are impossible. Now, that said- the IMU can detect movement based on the gyros and attempt to calculate what it thinks is the orientation of the quad relative to course direction, but that is complex and subject to accumulation errors. I suspect the Mavic indeed has these calculations, but the compass is still an integral sensor to resolving what is actually going on.
Airplanes rarely fly backwards or sideways, quads do. Compass is essential for quads. Not so much for airplanes that are not capable of flying backwards or sideways.