I mean it is decentralised, there is no single central point controlling the fediverse, that’s decentralised by definition. Yeah isn’t not decentralised to the point of being fully p2p, with everyone hosting their own data, but that’s just a different kind of/approach to decentralisation which has its pros and cons. Decentralised groups cooperating is still very much decentralisation by definition.
No one has to interfere with your data if you host your own instance after all, that’s always an option.