From what I have found, the answer probably lies in the following:
import inspect
print(inspect.getfullargspec(os_chdir))
# Prints FullArgSpec(args=['dir'], varargs=None, varkw=None, defaults=None,
# kwonlyargs=[], kwonlydefaults=None, annotations={})
print(inspect.getfullargspec(os.chdir))
# Raises ValueError: no signature found for builtin <built-in function chdir>
That is, unittest.mock.create_autospec()
has no way of finding the signature for such built-in functions.