So what would stop them from shadow profiling you by scraping content, or using a different domain? Most lemmy instances are configured to federate with a blocklist, meaning any unblocked instance can download data. Facebook can just make an instance under a different domain and download the data that way. Or they can just scrape user data from the web facing interface.
Posts and comments on lemmy are public. If facebook wants your publicly accessible data from the fediverse, de-federating from threads isn't going to stop them.