They are a separate rotational mass coupled to a rigid body by frictionless bearings. The fuselage IS NOT coupled to the treadmill because is is simply resting upon it. Remove the wheels and cover the treadmill in a perfect lubricant that cancels friction and the treadmill will move beneath the fuselage sitting atop it without disturbing the mass at rest! Allow the propeller to start moving and you get motion out of the fuselage that will eventually allow flight regardless of how fast the treadmill is moving because it has no effect on the plane anyway.

Come on, man...