That's not quite what the parent is trying to say. Assume that int is not None. Create a type called T that is int|None - a term of type T can be an int, or it can be None.
Now have a function whose argument is T|None. Substitute in the definition of T and you get (int|None)|None. If the function's argument is None, what does that mean? Was it given a T that happened to be None, or was it not given a T? Nobody knows.
Now have a function whose argument is T|None. Substitute in the definition of T and you get (int|None)|None. If the function's argument is None, what does that mean? Was it given a T that happened to be None, or was it not given a T? Nobody knows.