Also, ships have no need for particles to move. Their movement is based upon that little law that states "every action has an equal and opposing reaction". The engine propels back which in turn propels the ship forward.
It's a pretty simple concept. Basically, if you shoot stuff in one direction you will move in the opposite direction.
I feel like you failed to read his post correctly because he's a Nov10, but it's possible you just don't understand physics-related words =.="
The drag force he was referring to means that the particles impede ships' movement, not let it happen. If there were literally no particles in space (and the universe could still exist without all matter exploding from diffusion–oh, wait, that already happened, and that's why there are particles in space), then a ship, once far enough out of notable gravitational fields, could be propelled once. And that would, eventually, make it travel an infinite distance at a constant rate.
That doesn't happen.