Think of this way.
Satellites are solar powered.
Satellites also position themselves with thrusters.
Satellites are subject to orbital decay. In order to maintain a stable orbit, they must periodically use their thrusters to reposition themselves. Thrusters have a limited supply of propellant. When that propellant is completely used up. the satellite eventually falls into the atmosphere and burns up. (the effects of atmospheric drag eventually catch up to objects in LEO. One rather famous example of this is Skylab )
Even ion engines require reaction mass. The solar sail concept avoids this problem. As long as the sail survives, it can move in reaction to the sun, or a ground based laser, depending on how the sail is designed. It’s not pedantic, because this not a minor detail. Rather, it is the entire point of the exercise.
We don’t have Dean drives. If they worked, space travel would be far simpler.
