This is correct for right now, but data sharing is a feature we're currently in the process of wrapping up and getting reviewed by an independent security team. So this will be possible in the near future
To give an example: The indiehackers website uses firebase as its data store. It's a blog/forum, so posts and comments made by individual users can be seen by people who don't have accounts. Is this a use case you plan to support in the future?
If not, the potential use cases are much narrower than firebase. Not necessarily a bad thing but I'm just trying to understand the product.
Edit: technically you could do something hacky like share the database with a user who broadcasts the data somewhere. But I don't think you'd want to use Userbase if you want to store lots of data that anyone would be able to see