From what I understand they're basically doing this as an abstraction layer so as to support both Gtk3 and 4? I'm still not sure why it needs to involve a browser though..
>Ideally I would use an embedded Wayland compositor widget instead of a webview and broadway, but unfortunately there is only an old non working prototype called Wakefield and finishing it is a project on its own. In any case replacing the webview with a wayland compositor widget would be trivial.