Given that the universe is 13.8 billion years old, 40,000 years is nothing.
The presumption that other putative galactic civilizations start at nearly exactly the same time as us is implausible, especially considering more than 9 billion years passed before the solar system even formed.
Sure, but we'd see the markers on timescales relevant to us. The gap between the light cone and actual velocity is the critical difference between a kugelblitz and an invasion: you could conceivably conceal the former but not the latter.
I'm pointing out that any scenario that requires this synchronization is inherently implausible. So if we don't see the markers, trying to say it's because there are lots of civilizations but they just happened to pop up in synchrony with us is not a plausible theory.
The presumption that other putative galactic civilizations start at nearly exactly the same time as us is implausible, especially considering more than 9 billion years passed before the solar system even formed.