Maybe "dialect" would be more accurate than "superset"? I don't think Cython is technically a superset of Python, since I think runtime metaprogramming features like __dict__ and monkey-patching are significantly altered or restricted?
Cython is a reification of the interpretation of a python program. Ie it converts the python code into the equivalent CPython API calls (which are all in C) thereby allowing the developer to interperse real C code. Anything you could do in python you could technically do in Cython, although it would be much more verbose.
Yeah, I was going to make a similar comment. It's a dialect of CPython, and certainly there are extensions required to make it usable. But I'm not sure it is a strict superset of the full Python language.