It's inertia, INTBH. Those 20k+ pilots must have flown commercially, i.e. heavier planes, where the effect can be seen. Although like you say the craft is moving within a practically uniform parcel of air, Newton's laws still apply. The inertial system is the spheres, so the physics is there.
I have no claim how pronounced or measurable the effect is on those beauties with low wing loading, but our aircraft do show. Level flight, fixed thrust, steady heading with 90° cross-wind: there is a difference in which way you'd turn.
Last edited by FlightDetent; 11th July 2018 at 17:26.