Censorship only becomes a problem when it is _unavoidable_, that is, you cannot choose to live outside of some entity's censorship influence. This is where things get complicated when you have giant entities like Facebook or Twitter, which are for many people difficult to avoid (and there certainly isn't an alternative to them).
The problem arises when these entities need to choose which messages to show you. Even without putting any conscious bias in this algorithm, there is a system there which prioritizes some messages over others in a completely opaque way, which is often game-able (bad-faith actors can abuse the system to spread their propaganda, for instance). So moderation is not merely necessary but _unavoidable_, simply because there is more content than you can be presented with, and unfortunately the huge power of moderating large social media networks rests in the hands of a handful of engineers.
Decentralization is an interesting solution here because it would make the social graph and the recommender system separate entities, with clients being able to choose which recommender/moderation system they subscribe to. The need for censorship wouldn't be gone, but with people being able to choose their own censor, the massive power imbalance is not quite as bad.
