Decentralization isn’t done to hide the author, federating content works because the content is spread beyond a central owner. I don’t know if you ever used a peer-2-peer network like you do when you torrent a movie, but the concept is very similar. It is harder to censor something because you have more places you need to censor.

Imagine you are in a country where a lot of information is censored and you want to spread a message. Would you pick 1 giant billboard in the city center or would you make a bunch of leaflets you secretly hand out to someone you trust, hoping they will give the information along to someone they trust etc? Obviously, one giant billboard is easier to take down by the censoring government. That is why decentralisation does in fact work against censorship.

Anonymity or ‘layers of privacy’ are useful if you don’t want to be caught as the author of the message. In that case it is not about running the instance over Tor, but accessing the instance over Tor. You wouldn’t even need to use tor if you can trust your computer isn’t infected and you acces the instance through a VPN and remove all new data (e.g. cookies) from your pc before you disconnect your vpn.