ActivityPub, which is like the "language" that makes the Fediverse work, doesn't have special rules to keep things private. It's more focused on helping different parts of the Fediverse talk to each other. ActivityPub doesn't have special rules to keep your messages private, so we need to be careful about what we share and who can hear us.

Imagine you have a sandcastle on the beach, and you want to keep it just for yourself and your friends. But sometimes, there are sneaky seagulls that come along and try to take your sandcastle when you're not looking. Even if you build a strong wall around your sandcastle (like ActivityPub), these sneaky seagulls (called scrapers) can still fly over the wall and take your sandcastle away. So, it's important to be careful with what you share, even if there are walls to protect your things.

And defederation doesn’t necessarily make your posts private either. You might think because you built another wall for your sandcastle that it will be safe. But sneaky seagulls can still fly over to your wall and see your sandcastle, even if you’ve created a barrier to entry (defederation) that you believe keeps them away.

So, just building a wall doesn't always keep your sandcastle (or your privacy) completely safe from the seagulls (or people who want to see your things). They have wings. They can fly over it.