Not entirely. It is possible to see posts from people you're not following. I'm not sure how Mastodon does it exactly (I'm not on Mastodon, but the Fediverse is

Not entirely. It is possible to see posts from people you're not following. I'm not sure how Mastodon does it exactly (I'm not on Mastodon, but the Fediverse is more than just Mastodon), but I think it also has a tab where you can see other content. In the end, if any user on a server follows the poster, the post will end up on that server, and it's up to the server to decide who gets to see it, which could be everybody.