What’s the selfhosted component of this?

Self hosting your own private P2P voice service.

Optionally use your device as a Audio relay for group calls, in which case you become the ‘server’ to all connected clients.

Self hosting […] P2P

You do realise that’s a contradiction.

Unless you’re hosting a TOR node (which is outside of the scope here and, depending on the node and jurisdiction, extremely risky), there’s nothing here that’s relevant to self-hosting.

I don’t agree with that. Both sides are acting as a server and a client, connecting via a onion service to either parties rendezvous. And then when you include the fact that you can become a relay, that is clearly self hosting a server in a pure sense.

There are no exit nodes involved in onion services. It all stays within the network.