Just because it's a desktop client doesn't mean it's not using webrtc. WebRTC is a much broader standard than "in-browser videoconferencing".
The main thing is that it's an open(ish) standard built around peer-to-peer low latency connections, with a lot of software already built up around using it.
[edit] Ah I guess I'm wrong they use a proprietary thing outside the browser after all that they call BUD. https://blog.parsec.app/game-streaming-tech-in-the-browser-w...
Still I suspect that there's some cross-pollination there.
