Yes, it seems that every user already has a private|public key pair. It's just that instances are apparently storing users' private keys. And that, for what it's worth, seems like a horrible idea.

But whatever, if users had control of their private keys, they could easily move. As long as routing depended on public key hash (as with Tor .onion sites) instead of user@instance. More of a web model, instead of an email model ;)

That seems like an overly complex scheme to solve a problem that XMPP solved years ago with SRV records.