If life has formed on these other Earths, it has had, on average, 2 billion years longer to evolve than we have had. This is the fact that has Hawking worried.
If you imagine that the pace of biological and technological evolution on these other Earths is about the same as it has been on our Earth, then alien civilisations are on average two billion years more advanced than we are.
To put that time frame into perspective, two billion years ago, our ancestors on Earth were amoebas or parameciums – single-celled eukaryotes of some kind.
I agree with Hawking that we should keep our heads low. But we have already opened our mouths and started coughing – betraying our existence to any big-eared aliens.