They have a sandbox environment federating with 3rd party servers where other devs can participate in testing, and in the main public beta environment they just switched away from one main server to like a dozen (still no 3rd party there) and moved user accounts around, so they can test the federation code for stuff like performance and effects of account migrations, etc, in a live environment.
They’ve said they won’t open up federation with 3rd party servers on the main environment until they have moderation tools which can handle it, so they’re working on that also now.