Centralisation in this instance refers to control over the network and standard itself rather than control over what's posted on it. There's no single authority that can unilaterally change how every Fediverse instance and system works - for example, there isn't anyone who can decree that from now on Lemmy will no longer allow connections from Canada, or that nobody is allowed to post pictures of capybaras any more.
It's intended to prevent a /u/spez or Elon Musk situation where one asshole can bring down the entire ecosystem built around an API. Nothing stops anyone else from hosting their own instance if they dislike lemmy.world, whereas if you don't like Twitter, you can't just host your own copy of it.