Maybe I'm completely misinterpreting it... but my impression is that if my Mastodon instance defederates Threads, then I cannot interact, in any way, with anyone on Threads from my account on the defederating instance.

I'm on mas.to. The admin already publicly stated that Meta invited them to talks. They declined. And they even blocked Meta's access to anything mas.to before Threads was even a thing. Defederation is probably a foregone conclusion there.