Anything made in that period is inaccessible from our local instance,
Why would it be? It makes no sense that age matters. I just arbitrarily searched for a community that was newly created on one instance from another instance, and it was found. I had to wait around 3 seconds because the check is done in realtime.
E.g. !Google@thelemmy.club was created just 1 week ago. Yet it’s found in just a few seconds by other instances.