JSON is not "fully typed". It just happens to have different syntax for strings, numbers, and booleans. But the application code still needs to come up with a way to distinguish between timestamps, enums, different object types, etc.

XML uses the same syntax for strings, integers, and booleans, but it has mature schema/typing tools that make it easy to apply more precise typing, which you'd want to do anyway to identify timestamps, enums, and different object types.