Generally that is true, but when you access a remote community for the first time, Lemmy attempts to backfill several posts from the community. This is limited to only posts, so comments and votes are not included in that. You can also "resolve" a post (or comments for that matter) on an instance from its fedilink (the colorful icon you see next to posts and comments), so when someone links to something elsewhere, a lot of apps will try to open (by resolving) it on the current instance instead, which can also result in posts or comments showing up, even when there isn't a subscriber. Resolving can also be done manually by entering the URL in the search. This seems to not always be that reliable to work on the first try though, so it can help to try again if you have trouble resolving something on the first attempt.

I think there is also something updating community information in the background from time to time, I'm not sure if that only happens under certain conditions or in regular intervals, and I'm not sure whether that fetches new posts at that point either. If it does, it could explain new posts appearing at daily or so interval but without any comments and votes. Backfill should probably only happen initially when discovering the community for the first time though.