The difference is that rather than using the admin interface that ships with WordPress, we're encouraging usage of this new interface. It still talks to WordPress via a PHP-powered REST API.
In short, it's a new _interface_ for WordPress, not a replacement for WordPress itself.
> https://wpcalypso.wordpress.com/devdocs/design Anyone know if this is a bug, or part of the new approach?
Bug. That's one of our internal staging sites and the devdocs are stripped when pushed out to the production URL.
I've opened an issue for you: https://github.com/Automattic/wp-calypso/issues/577
EDIT: Not private! Just requires a login from any WordPress.com account. There's discussion going on on that ticket to move it somewhere that doesn't require a login but it might not be possible.