Accounts, yes. Posts, not necessarily. I joined during the great Reddexodus, when the influx caused several instances to go down temporarily. What I recall happening was the communities that were mirrored to other instances still had accessible posts and comments, but they were essentially frozen? Like you couldn’t contribute any more to them without the host instance coming back online.

I think the way it works is if you are the first to subscribe to a community from a non-local instance, its content gets synced to yours, which adds some resilience in case that the remote instance goes down. At least that’s my impression of how it works.