The huge instances (mstdn.social, if not also mastodon.online) are also threatened with defederation because their moderation isn't scaling upward fast enough. That's not a problem of policy but resources. Brandolini's Law can affect any instance of Mastodon, even the ones leading the calls for defederation. I think Mastodon is better technologically than Twitter, but the fact remains that zergs are an effective tactic. The problem can be automated, but Mastodon doesn't really have the automation to moderate yet, and that's a long term problem waiting to happen.