The thing is, community on the other instance has to be “discovered” by your instance first. That happens when you search for its url from your instance. From that point, it’s added to the list of communities under All on your instance, and new posts are synchronized.

But posts on that community from before it was discovered don’t seem to be synced, only new ones. That’s a known issue and I’m not sure of they’re working on it.

The number of subscribers you see is just from your instance.