[incompatible]

You'd have to have a lot of trust that the one instance wouldn't get enshittified and end up as another X / Truth Social / BlueSky.

Different instances can also have different rules, different moderation and different federation.

Edit: and exist in different legal jurisdictions, and also be harder to ban or regulate.

[akho]

Having many instances is not the same as federation.

[incompatible]

Isn't one a subset of the other? A system with federation must have multiple instances, but a system with multiple instances doesn't need to federate (in the sense of information passing between independently managed instances.)