I would like to think that a sophisticated civilization would understand what was happening.

I also giggle at the idea of a highly advanced civilization arriving at earth to tell us humans exactly what they think of someone firing a relativistic shotgun at their planet. Even if it was only loaded with bird shot.

Would they even notice it though? Normal bolides of similar energy happen fairly frequently (see https://catalog.data.gov/dataset/fireball-and-bolide-reports for an example). And even if they did, would they interpret it as a hostile act when similar events happen naturally without ill effect? It would mean the "attacker" posses the technology to accelerate materials to appreciable fractions of c while simultaneously not knowing that the impacts wouldn't do any damage to the target.

Should we similarly be afraid of making radio broadcasts lest our intended recipients interpret our beaming of energy as a hostile act? A somewhat similar concept is actually mentioned in the novel Blindsight. You could even go full Dark Forest and say that we're a threat simply by existing all the while brazenly advertising our presence through the oxygen in the atmosphere.

If there were a technological civilization there we'd have heard their radio broadcasts. [sad-face] - because I wish we had. It'd be nice to have neighbours.

Interesting to think about but my gut is telling me it's not even worth planning for. It should be an infinitesimal chance of impacting a planet.