Are you referring to the WebSocket API that the Firebase SDKs use internally? It doesn't seem to be documented anywhere so I guess it's only slightly better than scraping HTML ;)
Thanks for the tip, I actually just figured that out myself a few minutes ago. Should be good enough until a proper SDK arrives.
With access to a Firebase SDK the only major additions the API needs to become a viable replacement for existing read-only client apps would be support for all the other lists apart from top stories (new, show, show new, ask, jobs, best, active) and more than 100 items for each. For apps that need write access I'd suggest keeping the current website on a separate subdomain until that is implemented into the API.