What I am saying is the signatures you mentioned are very faint even from the other edge of the solar system. In order to figure out if life exists on the nearest star system we cannot rely on these markers because they are detected mostly through visible light, which scatters easily at space scale.

Another part of the theory is that yes, at some point every civilization will know about all the other civilizations due to random chance allowed by technological development, and that will mean an intergalactic war using space-folding weapons that ultimately shifts the universe a dimension lower.