A state-level actor can get the Signal key either covertly or by simply marching into the Signal offices with either a warrant or (if that fails) guns. Now, whether that will actually happen is a secondary issue -- but I submit that you have a mistaken conception of what a "state-level actor" means in a threat model. The fact that Google Inc doesn't hold the necessary keys but Signal LLC does is not a meaningful distinction to a state-level actor.

That isn't to say "all crypto is hopeless", simply that you shouldn't consider Signal to be state-level actor proof.