Well, Einstein did not say faster than light travel was impossible. The math does not say it is impossible.
What is clear is that it’s profoundly certain that one can not exceed the speed of light in a given reference frame.
What many have postulated is that the “trick” is to step outside the box, or in this case to somehow remove a vessel from a reference frame. IF one blocks or shields gravity and therefore inertia, in theory there would be almost no limit to the upper speed one could attain.
The Albicurie warp drive is one proposal.