I do not agree with the people wanting to control other servers by trying to force defederating from threads. Independent admins running their own server is what the Fediverse is built upon.
As long as authorized fetch is implemented (and correctly), intermediaries can't "leak" messages out anyways. If Threads wanted to read the contents of a boost, they would have to ask your server for that, and your server can tell them to screw off.
Does kbin or Lemmy implement authorized fetch? If they don't they should start working on it. And consider enabling it by default. I know versions of Lemmy >= 0.18 can talk to GTS (which enforces AF) so there is partial support for it. And nobody runs 0.17 because of how inefficient it is, so that won't be too big of a backwards incompatibility issue. No idea how it works on kbin land here, but it should be implemented ASAP if only so that any future enforcement won't break backwards compatibility.