> The value of A.b can, incidentally be accessed via obj.b because obj.b does not exist and the lookup path for instance attributes includes the class. [...] the += operator, which calls the __iadd__ method on the left value or acts as add and then assign to the name on the left, if that name doesn't reference a value that supports __iadd__.

That is not correct. += will always perform the assignment, hence the classic oddball concatenation:

    >>> v = ([],)
    >>> v
    ([],)
    >>> v[0] += [1]
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: 'tuple' object does not support item assignment
    >>> v
    ([1],)

So `obj.b += [2]` desugars to something along the lines of:

    l = obj.b
    l.extend(2)
    obj.b = l

Hence if you call vars() on the instance or access its __dict__, you will see that the instance does have an intrinsic "b" attribute:

    >>> class A: b = [1]
    ... 
    >>> a = A()
    >>> a.b += (2,)
    >>> vars(a), a.__dict__
    ({'b': [1, 2]}, {'b': [1, 2]})

And if you re-set the attribute on the class, it won't affect the instance, which it would if the instance delegated to the class:

    >>> A.b = [5]
    >>> a.b
    [1, 2]