Thus it is not at all true that that we ignore the possibility of many quiet civs.
But that’s not the claim of the quoted text, which is explicitly about quiet expansionist aliens (e.g. expanding as far and wide as loud expansionist ones). The model does seem to ignore those (and such quiet expansionists might have no borders detectable by us).
[Edit: For more on quiet expansionist civilizations, see the more recent post “Silent cosmic rulers”.]
But that’s not the claim of the quoted text, which is explicitly about quiet expansionist aliens (e.g. expanding as far and wide as loud expansionist ones). The model does seem to ignore those (and such quiet expansionists might have no borders detectable by us).
[Edit: For more on quiet expansionist civilizations, see the more recent post “Silent cosmic rulers”.]