Defederating actually does stop Meta from accessing data (at least through ActivityPub) if you enable AUTHORIZED_FETCH / similar. That setting requires remote instances to authenticate themselves, which prevents blocked instances from querying anything. IIRC, Lemmy either already supports or plans to support that same feature.

Meta could, of course, just use web scraping, but that can be prevented with DISALLOW_UNAUTHENTICATED_API_ACCESS. Although admittedly, I don't think Lemmy has this feature yet.