It's true that the mechanism is different, but this still works (to a limited extent) even without any external object to transfer momentum to, just using the m

It's true that the mechanism is different, but this still works (to a limited extent) even without any external object to transfer momentum to, just using the momentum of the photons themselves. I can't be bothered to draw a diagram, but imagine that the sun is to the "north", and the solar sail is at a 45-degree angle running NE/SW. Incoming photons heading south reflect off the sail and leave to the west. So the photons experience a net momentum change to the NW, pushing the sail to the SE, which is not directly away from the sun.