That example is easy to solve because it is not using any of the provided techniques that makes it more difficult for robots to solve the CAPTCHA: "changing the HTML background color based on: mouse cursor hovering, previous state or input or shared secret"

The purpose of the example is to give you a basis on which you could implement an effective CAPTCHA.

The claim of "difficult or even completely impossible for robots" applies to CAPTCHAs using the above techniques, which are not used in the example.