One but difference of this approach over just using the original library directly is that now you have a type system that can enforce more invariants than it is

One but difference of this approach over just using the original library directly is that now you have a type system that can enforce more invariants than it is possible in C and that the library was previously already relying on. If the (now reduced) API surface happens to still expose a bug, it can both be fixed in the library and/or protected against in the API.