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.
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.