And the example below that one shows how to hydrate declarative shadow roots with JS custom elements, so the path to progressive enhancement should be clear.
I say alleged because I haven't had the chance to try it beyond a toy example.
You've ignored the part where the declarative template is immediately attached to its parent as a shadow root by the parser, and its siblings become the slot children. You don't need to instance or register anything with JS there, just copy-paste the template (admittedly lame, but kinda the same repetition as SSR today, just less involved and with a shadow root).
> This won't appear in your page until you grab a reference to it with JavaScript and then append it to the DOM