It's a public demo instance that's not unique to you.
Anything you can do or see, anyone else can see or do.
If you create a knowledge base article on the demo with your credit card information in it, that'll be visibile to everyone.
Likewise if you add in the details for an actual server that exists, someone can see the connection info for it and perform actions on it through the FOSSBilling interface.
That is why you shouldn't even be attempting to add a server to that demo instance.
We will be getting that functionality properly blocked in the future