I would say that for an action to be considered censorship in the strictest sense, it would need to be the suppression of information as imposed and enforced by a monopolistic authority.

If the State were to declare a book banned, that would be censorship because the State establishes itself as the single totalising authority over the people in the territory it governs. Should you contravene that ruling and possess the material in question, you’re opening yourself up to the threat of violence until you start respecting it. You’re not able to opt-out, the single authority imposes itself and its ruling on you.

Meanwhile, on federated social media there are many concurrently operating instances with different rulesets and federations. If the instance you’re part of decides to defederate with another, then you can move to another instance which continues to federate with the defederated instance in question if you’re unhappy with the decision. You’re able to opt-out of that ruling without consequence.

Plus, even if you decide not to move instance, the content hosted by the defederated instance will still be available through the instance itself.

Defederation doesn’t meaningfully suppress information, whereas censorship does.