From what I understand about the fediverse, everything that Meta already knows about you and your content is going to remain the same. Federated or not, they will only have access to info you make public, and how much that is is entirely up to your instance. Even if Meta never wanted to join and federate with others, they could still scrape the same info and use as they see fit for training AI, duplicating posts or whatever.
So basically you don't have to do anything, Meta cannot demand any extra data from you that your instance owner does not want to share. If you are worried your instance owner might do some deal with Meta for your data, then you can better control that with self hosting.
Unless i misunderstood, I believe the point of this post wasn't talking about defederating, it is asking about protecting the users data from Meta if federation happen, and for that point it is what I said, his data is as protected as before, federated or not, Meta won't get access to any extra data from being federated than it already has access to now.
Defederating is another entire discussion that is really up to the instance admin, self hosting is the only way to really guarantee anything if you want that much deep control of your data and fediverse experience.