I think you’re limited to when the first person on your instance subscribed to a community on another instance - from that point on, all posts/votes/comments will shared with your instance and be visible, but before that, I’m not sure (you might get the post, but not the comments, or you might not get the post at all).
I wanted to comment on a couple of 2-year old posts I found via google - when searching from my instance, I found one post (but no comments), but not the other post. When I added my comment and re-viewed it through google, it was there alongside the 2-year old ones.
If lots of people are signed up to lots of different instances, then a ‘fediverse’ will grow as expected, but if a majority are congregated on a few big ones, it limits the potential for spread.
I’m still trying to understand all this, so I might be wrong.