You can't really compare Confluence with GitHub. Confluence supports such things as graphical editing, a full, rich feedback loop, linking, attachments, history, fulltext search, PDF export and the like.
Have a quick look at the documentation: https://www.atlassian.com/software/confluence
The trouble with GitHub as a kind of CMS is that there's no graphical editor (to my knowledge), linking and renames aren't automatically handled, so one rename will break your existing site, there's absolutely no automatic hierarchy and navigation, and for the most part, the feedback loop from code to publish is hours or days rather than seconds - most humans like to see something instantly and won't bother if it involves having to manually code HTML, send for publish, get published, etc.
Anyway, until I get the OK from one of the core team, which doesn't look like it's forthcoming, I won't be emailing Atlassian.
Thanks for all the thoughts, ladies and gents!