One aspect we can see from the current Threads integration is if a Threads user @-mentions a non-federated Threads user in their post, that post will not be federated to ActivityPub. So if users aren’t opted-in by default, we're going to see a patchwork of missing posts and broken reply-threads rendering the whole thing pointless / @mosseri
EDIT: looks like that may be fixed, now? Threads posts with mentions are now retroactively showing up /cc @jessel