They're shadowing builtins. Python's list type can't be referenced by its name in the defined sum function and Python's sum function can't be used in or after the new function's definition.

One could argue that in a source meant to teach Python, it shouldn't use examples that don't follow best practices. I feel that as a resource to, from the page, "learn how to write code, the basics and see examples" that it doesn't really matter too much.