Within the confines of the story:
* No star that has been visited by starline has ever been seen from another, which implies a vastly larger universe than can be seen from a given lightcone. Basically, granting the slightly cryptographic assumption that travel between stars is impossible.
* The weapon is truly effective: works as advertised.

Any disagreement with that would have to say why """ 'Assume there is no god, then...' "But there _is_ a god!" """ fallacy doesn't apply here.

The threat of a nova feels like a more interesting avenue than the mere detonation.