To me this sounds like us de-federating them early on to avoid them de-federating us. It’s an open framework enabling multi domain interoperability. As long as fediverse rules aren’t violated no one should get defederated imo
Again tbh, I don’t really think Meta needs Fediverse. They already have Facebook and Instagram. All they need is add one link and they’ll have way more users than the Fediverse has in a matter of hours
One issue with emails is that it's actually very difficult to self host email servers now as most of the bigger servers would automatically block unknown servers due to spam