xkcd

Not exactly - what was proven is that there's no way to distinguish between inertial frames of reference. There could be a universal frame of reference, which would most likely be the average velocity of all things in the universe. There's not much point in making a distinction in most cases, because if you can't detect it, it might as well not exist - but since we're making up time travel, we might as well make up a universal frame of reference, it doesn't break anything time travel hasn't already broken...