So their calculation is actually for the number of civilizations in the galaxy that are at exactly the same level of technological development that we are. Even

So their calculation is actually for the number of civilizations in the galaxy that are at exactly the same level of technological development that we are. Even assuming that some percentage of civilizations annihilate themselves beyond recovery there must still be a very long tail of ones that only do so at a later date as well as those that do not and those that recover at some later time.