Indeed, it's a pretty strong argument that the most prudent (in the context of the continuing survival of your own civilization) reaction to discovering a developing civilization in another solar system is to preemptively destroy it. Also, The Algebraist by Iain M. Banks. :)
Unless you assume that cooperation is more likely than conflict. Maybe conflict is more common than usual on Earth. Imagine for example a group of worlds that were able to survive only due to cooperation. On encountering us they would likely lean towards cooperation as that was their experience. We might lean towards conflict but our experience tells us little about the average experience of other inhabitants of our galaxy.
The scary thing about the xenophobic solution to the Fermi paradox is that there might be lots of nice civiliations around that do want to cooperate - but it could potentially only take a few very aggressive, advanced xenophobic bullies lobbing large objects at anyone they spot before the threshold for everyone else to stay in their metaphorical foxholes hiding until/unless they have very good reason to assume they have established a clear advantage.
While we only have a sample of 1 (Earth), we have do millions of samples of interspecies behavior that lean strongly to conflict, and often the cooperation isn't necessarily pleasant for one species either.
What if the first message you receive is from a peaceful and advanced civilization, and you reply with "Hi, we are earth, send your teleportation devices here please", and within the 60 years that the round trip message has taken to be received, a military junta has taken over the civilization and decided to exterminate all humans because our exponentially accelerating technology will make us a threat to their hegemony in another 60 years? :)
