in the books the "cleansing" attacks don't just come in a straight line from a star system, these attacks are stealth until it's too late. thus, the attacker cannot be identified by others.
But how do you know space isn't filled with a network of probes to detect just that? It seems to me you need to be confident you're first on the scene, or at least that the Precursors don't have such a huge lead on you that your diversions are useless.
Edit: Also it seems that by a strange argument combining 1) any civilization earlier than you is likely to be massively more technologically advanced and you cannot hide and 2) you exist, that any civilization could assume they are either first or first enough, which also seems to break the dark forest.
Edit: Also it seems that by a strange argument combining 1) any civilization earlier than you is likely to be massively more technologically advanced and you cannot hide and 2) you exist, that any civilization could assume they are either first or first enough, which also seems to break the dark forest.