This isn't really true. In GR, you can in principle always distinguish acceleration from gravity over finite stretches of spacetime by measuring the tidal forces. ...

Yes, I was just listing an offhand example of an implication of GR and I didn't bother to specify it to full precision. My point was just that in order for a certain implication to be falsified (specifically, that there is no fact of the matter as to e.g. what the velocity of the universe is), you would need the laws of the universe to change, not just a refinement in the GR model.