They've said very clearly this is not an optimal or even a permanent solution.
Lemmy/etc are lacking moderation tools that other platforms like Mastodon have. They are not seeking isolation from the rest of the fediverse, but they do need more effective moderation ability, and right now the only viable solution they could settle on is temporary de-federation.
That said, obviously lemmy.world doesn't have to just sit in a weird half-federated state. It's just worth understanding why they're doing what they're doing.
Interesting. I might just be blind. Thanks for the update.