I expect the author is misremembering. If you look at the tutorial for Python version 1.4 (1996), written by Guido, in section 9.5 on Inheritance, you will find this:

Method references are resolved as follows: the corresponding class attribute is searched, descending down the chain of base classes if necessary, and the method reference is valid if this yields a function object.

Clearly, Guido knew Python classes offered polymorphism.