I think a potential difference here is that a substantial part of the existing Fediverse won't care if we break compatibility with Threads. Many will actively welcome it, so there's potentially less pressure to yield if the make changes people don't like.
The Mastodon corner of the fediverse is also ridiculously more well run and diverse than xmpp outside the big players ever was.
Like, when threads joins it's far far more likely to be a net contributor of spam and abuse towards the rest of the network because the people who run Mastodon instances generally actually care.
Even Mastodon.social (the biggest instance currently) routinely gets silenced or blocked temporarily by other instances when it lets spam get out of control, and that is generally considered a good thing by users.
Honestly that's gonna be the main reason threads gets defederated after the first round of ideological blocks: self-defence against abuse.
