Point is you cannot know if they are an upstart (whatever upstart means). It can be misinterpretation, it can be camoflage, it can be anything. But once you rain death you're better be prepared to be grateful for what you are about to receive back.
You wouldn't be able to know this over the vast distances of the universe.
People are arguing two contradictory things: these are unfathomable alien civilizations with motivation and timescale we cannot comprehend, but we would perfectly understand their tech level, location, and capability of striking back.
It doesn't add up. It's a scifi premise needed for interesting plot conflict.