That is true, but due to limit laws the earth could be constantly approaching the speed of light without ever actually reaching it, also the earth contains much less mass as a flat disk rather than a gigantic ball that you round earthers are trying to push on us, and since it's in a vacuum the object would be able to be accelerated with very little thrust needed (think twin ion engines). As for the means of propulsion it's unclear, solar winds, rocks falling off of the bottom side of the earth, it's really anyones guess.