CPython only makes these methods class method when a class created. If you set __class_getitem__ method after the creation it doesn't work unless you use classmethod decorator manually.

>>> class B:
...     pass
... 
>>> def y(*a, **k):
...     return a, k
... 
>>> B.__class_getitem__ = y 
>>> B[int]
((<class 'int'>,), {})
>>> B.__class_getitem__ = classmethod(y)
>>> B[int]
((<class '__main__.B'>, <class 'int'>), {})