As another perspective, by documenting that API, they're promising to retain backward compat and the documentation burden that comes with it. As long as it remains an internal API, they can fiddle with it as fast as they can get their old codebase off of it

Since their codebase is in the open, and there's already at least one other Open Source implementation, that's likely about as "clear/open" as its going to get unless this newfound $100M spawns an API team or something