https://raw.githubusercontent.com/relax/relax/gh-pages/asset... shows the key features are WYSIWYG-style building of entire HTML pages.

This is not a Content Management System, but a Page Management System. A CMS works with structured content and the relations between those pieces of structured content. It allows you to model the data it needs to store, receive (forms) and render. So you can then reuse, combine and display that data in any way you need.

(Many projects/people conflate "page builder" with "CMS".)

That being said, this looks very nice :) There totally are use cases for an easy-to-use page builder :)