Here's a post about custom menu slides that might help you:
https://community.articulate.com/discussions/building-better-courses/create-a-custom-menu-slide
However, custom menu slides are typically used instead of the built-in menu. The course you linked to in review shows the built-in menu. And the navigation isn't restricted, so the user can click on any item in that menu at time. That defeats the purpose of a custom menu.
FYI: Here's more info about different ways to control navigation:
https://community.articulate.com/discussions/articulate-storyline/tip-controlling-the-next-button-101
