It doesn't matter. The point is that the only local-galaxy civilizations we would fail to detect for being "too far" would be those that were born only yesterd

It doesn't matter. The point is that the only local-galaxy civilizations we would fail to detect for being "too far" would be those that were born only yesterday, on the timescales involved. All things being equal this should be an irrelevant fraction of the total. A different matter is "too short lived", which is taken into account in the Drake equation (the last factor, L).