I have doubts also.

The issue is the same for NotImplemented, though the occurrence is much rarer, and similar for Ellipsis.

>>> NotImplemented.foo
Traceback (most recent call last):
  File "<pyshell#19>", line 1, in <module>
    NotImplemented.foo
AttributeError: 'NotImplementedType' object has no attribute 'foo'
>>> Ellipsis.foo
Traceback (most recent call last):
  File "<pyshell#20>", line 1, in <module>
    Ellipsis.foo
AttributeError: 'ellipsis' object has no attribute 'foo'

Replacing the type name with the object name works for this message, but not for the type errors.
  TypeError: unsupported operand type(s) for +: 'None' and 'int'
is wrong.

Replacing 'NoneType' with 'None' in error messages will break code that does something like "if 'NoneType' in err.args[0]" in an exception message.  The same replacement would have to be make in user code.  Fortunately, it would continue to work with older versions.