No since experientially we already know that we don't perceive the world as if all everett branches are computed.

In other words what is up for discovery is not 'not all everett branches are fully realized'....that's something from our apparent standpoint as belonging to a single such branch we could never actually know. All we could discover was that the collapse of the wavefunction is observable inside our world.

In other words nothing stops the aliens from simply not computing plenty of everett branches but leaving no trace in our observables to tell us that only one branch is actually real.