It's about the level of abstraction. Kuma's "Kubernetes Cluster" is very complicated but it serves no purpose to include its details in a blog post that describes Yari. Yari's diagram is more detailed than Kuma's, while Yari itself is simpler than Kuma.

Probably looks complicated because of the names of all the services. In a nutshell, the data lives on GitHub, gets automatically compiled into HTML and sent to a web server. They have basically taken the rendering out of the app server and into a CI task that does it all in advance.

Agreed... I'm not sure what of value is really left for non-contributing users on the site.

This is a similar tactic that MS has been using towards their documentation as well, so it's not so bad and for those most likely to contribute to technical documentation like this, the overlap with Github is significant enough.