There is an assumption that any changes or additions Threads may make to their implementation of ActivityPub beyond the official spec will break compatibility with other instances. It won't though, that's the point I was trying to make above.
Any additions they may want to make can absolutly be added on top of the existing official spec without breaking compatibility. Lemmy has downvotes but can still read comments and posts by Mastodon users. Mastodon users can post to Lemmy communities. You can see Pixelfed pictures on Kbin. Kbin posts can be read on Misskey. Misskey posts are visible on Mastodon.
All of these services have features that don't exist elsewhere, built outside of the existing spec, but the core content is all interoperable. Anything Threads may want to add can be done without destroying spec compatibility. Sure, they could still make a change that intentionally breaks compatibility, but why would they? Theres nothing in it for them. No one who's here is going to leave just because the Threads users are gone. The Threads users are already absent and we're all still here.