What point was that? If you don't join Threads, they don't have your data. They do have everything you publish to the Fediverse though, no matter what you do.
How? Everyone says some form of this, but never gives any details of how they will do that. Or how defederation will prevent that.
You are afraid that they will control the marketplace, and then turn around telling everyone to defederate, which just gives them a walled garden marketplace on a silver platter.