If you fetch a community that your instance hasn’t previously heard of, you can typically query the community’s ‘outbox’ collection to get recent posts. So in Lemmy, you get 50 old posts, and then - once someone has subscribed - new posts start coming in.

Different platforms have different formats for their outboxes - Lemmy uses Announce/Create/Page, a.gup.pe and PeerTube use Announce, with a URL that leads to a Note or Video, wordpress uses Create/Article. Because Lemmy already understands its own outbox format, it’s able to get old posts from other Lemmy instances. It doesn’t get old stuff for a.gup.pe, PeerTube, or wordpress though.

So you might be wondering what outbox format nodebb uses - to which the answer is none. The outbox leads nowhere useful (they’re in good company with MBIN on this). Anyway - this is why fetching a nodebb community won’t come with any of its existing posts (but - as mentioned - new stuff will come in for subscribers)