But here the movement is relative to two things. You're aware of your motion relative to the plane, and the headset is aware of its motion relative to earth's gravitational field. Einstein's elevator experiment only works when you're traveling on a (reversed) path that matches one that would be caused by gravitational force.
The other thing to think about is a lot of these systems use kalman filters and sensor fusion, so stuff like a compass will cause drift. They jsut didn't design the software to think about movement. It could be conmpensated for.