I think most people believe that defederating means your messages stop going to Facebook when infact it's the other way around. Only way to prevent Facebook from seeing what you're posting here is if they defederate with us which probably is easy to accomplish by having content on your instance that's agains Facebook's terms of service and that you refuse to take it down even if they threaten to defederate.
What defederating (if we do it) does achieve however is that it removes all Threads content from our communities which probably isn't a bad thing either