ActivityPub (the model that the fediverse uses for federation) is publishing-based, as the name implies. Like email, you're sending messages to a list of recipients. Usually that's your followers, people you mention, and the person you're replying to.
If the recipient list is empty, then your message won't leave your instance.

Threadiverse users don't really have to worry about this too much because communities act as relays, sending your posts to all of the community's followers as well. But microblogging instances don't have that luxury. If they don't have any followers, aren't writing a reply, and don't mention anyone... their post isn't federated anywhere.

It's also worth considering that only public data is federated. For example I wouldn't be able to recover my bookmarks from another instance, and it doesn't seem like Lemmy federates your list of subscriptions. Your posts may still exist elsewhere even when your instance goes down, but that's not necessarily the data people want to be able to recover.