Since their core product is an API, they've done a really good job building in all the levels of REST discussed in the article, including discoverability.
Any website that uses HTML forms (properly) is at the last level. Also, using the <a href ... > tag is also using HTML as the engine of application state transitions.
This article makes things out to be more complicated than they actually are. Just make decent HTML. REST was extracted from the web, it doesn't need to be imposed on it.
