They mean start posting, viewing and interacting in it.

Email has federated pretty reliably. Granted, it's nearly impossible to host your own server, because the rest will automatically assume you are only sending spam, but the well-known servers all federate quite nicely. Worst case scenario is that some organizations put a footer on outside messages warning that it's not internal.

Looks like data/account portability is in the works: https://swicg.github.io/activitypub-data-portability/lola

But at ATProto/Bluesky, account portability was a consideration from the start.