It just might be low energy photons providing the thrust
I don't see why this is so puzzling. If it works it is probably due to photons being ejected out of the 'nozzle', after all photons are the force carrier of the electromagnetic force. the photons don't have to be in the visible spectrum either, they could easily be microwave radiation.
If the sun's mass is kept from collapsing inwards by the light pressure of photons why can't a simple motor operate by using photons: energy produced by solar radiation converted into electricity and then further transformed down into microwave photons. it doesn't even violate Newton's third law!