It seems to me like a mischaracterization of JSON Schema to say you can't define a concrete type without actual data.
I am a very stupid individual so I could be misunderstanding the argument.
The thought I do get: JSON Schema primarily describes one main document (object/thing). And additionally defines named types (#/definitions/Student). But it's totally fine to just use the definitions for code generation.
The reference semantics of JSON Schema is quite powerful, a little bit like XML with XSD and all the different imports and addons.
The moment you have types referencing other types in a way that can become recursive in ANY way, the whole thing seems to explode.