The types are no less protectable by authorization policies than the data, although authorization is hard to get right anyways, all else the same this architecture doesn't worsen it much--perhaps just less reverse engineering required to exploit vulnerabilities you already had.
In Django I had code in every view ensuring that the user was not accessing another user's data. If I create an abstraction like a utility function or "model manager" to handle this, I have to remember to call it from every view. In Graphene, this logic would be in a resolver which is used across multiple views.
If serving up metadata about what the site already makes accessible via the API will get their data stolen, then the actual problem is they're not applying access controls to the data, not the introspection.