> [1] It means that civilizations should exist that are millions of years more technologically advanced than us;

> [2] and should be leaving telltale signatures across the sky that we'd likely have detected by now.

> [3] And the absence of those signs would be relatively strong evidence that life, while common, isn't long-lived.

> [4]Suggesting that our demise too, will come before too long.

So they were talking about the great filter, not alien invasions, which is what you appear to be replying to.