Yes, this is exactly the sticky issue we get into. And I'm wondering if lawyers would be able to make a case that using ActivityPub alone automatically gives your consent to have your data exist on an instance outside your own. Once they have data you've consented to give they can do with it as they please, essentially arguing you've become a consenting party when you consented to federation. I don't know the GDPR well enough to have any answers, but you can bet Meta lawyers do.
I don't think Facebook would be having high level NDA-protected talks with Mastodon people if they weren't trying to work all this out. And by work out, I mean how to monetize/data mine. I've been talking about this with people all day, many of whom didn't see a problem with this, but eventually all of them have had the lightbulb turn on when they realize the potential abuse Meta could do with/to ActivityPub.
If, by some miracle, Meta wants to be the good guy for a change, let them prove it. I would love to see defederation by default, and let Meta prove they're trustworthy to federate to. And even then, have a really itchy defederate trigger finger if they even hint at pulling another Cambridge Analytica fiasco. But getting everyone on-board with that is probably impossible, especially if Meta starts throwing money around.