Does this help?

Think of an airplane as two independent entities:

The airframe, and the rolling gear.

Any and all forces generated by a conveyor in an attempt to affect the perceived forward progress of the airframe will instead be enacted upon the rolling gear only.

The force the conveyor is trying to enact upon the airframe is lost due to the free-flowing wheels. The force is dissipated through the wheels, and never reaches the airframe.

The airframe, in essence, does not know what the conveyor belt is trying to do, and doesn't care.