The frontend repository above is only for the user-facing front-end website (aka the guardian.com). Other back-end components such as the content API and the content creation tool (Composer) are separate projects in their own private repos (not open source). That said the Guardian has many other open source projects on their GitHub org [1], including their image management tool for instance [2].
[1] https://github.com/guardian [2] https://github.com/guardian/grid