WordPress Calypso is an interface for WordPress.com that communicates to the classical WordPress background using a REST API.
The communications are done using a helper library called WPCOM.js, by default it is designed to use WordPress as a backend, but by modifying the interface to communicate to other backends will enable the use of Calypso with Medium, Contentful or Prismic.
These are just some options that you could use to enable scaling and truly separating the CMS: Dividing it to a content repository and an editing interface. Other tools like Drupal and eZ Platform are well equipped for REST communications and eZ Platform itself heavily utilises the REST API.
Learn more about truly decoupled CMSes with REST APIs.