A big part of that is true. It's mostly about Git. But it is also about GitHub, GitBook integrates well with GitHub repos, it links up your book to the issue tracker, watchers & stars as well.
Plus it's easy to host the HTML output of your books on GitHub thanks to their brilliantly simple gh-pages service.
AFAICS, none of this is about github. It's about git, the protocol, the versioned object store and markdown, the markup layer.