This is not true for App Engine. You can create a "staging" version of your app and access it at staging.my-app.appspot.com. It all runs on the same infrastructure that your main version runs on, and it's easy enough to make it admin-only. Once your satisfied that it's safe to deploy, it's two clicks to change which version is serving.
You may be the only managed Wordpress platform to do so, but App Engine has had this functionality for a long time.
You may be the only managed Wordpress platform to do so, but App Engine has had this functionality for a long time.