Yes. But the equipment on the back is radiating heat (roughly speaking) in every direction, as I understand it, so absent a reflector, the equipment would have

Yes. But the equipment on the back is radiating heat (roughly speaking) in every direction, as I understand it, so absent a reflector, the equipment would have a net zero effect on thrust. But because some of the heat is being reflected, it's contributing a non-zero amount of thrust.