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Defederating with Threads overview
(fedipact.veganism.social)
A community dedicated to fediverse news and discussion.
Fediverse is a portmanteau of "federation" and "universe".
Getting started on Fediverse;
I think it's just the fact that they've additionally signed this "pact" that says they'll never federate with them or anything else meta again. See the link at the bottom of the page.
It's mostly just symbolic, a sign of commitment.