Communicating between a Chrome packaged app and a Chrome extension?
Yes, that is possible. The code sample in the documentation you linked works for any combination of app and extension.
The extension documentation for chrome.runtime.sendMessage says:
Sends a single message to
onMessageevent listeners within the extension (or another extension/app).
Messaging works the same in both extensions and apps, and they seem to be fully compatible; simply use the ID for the destination extension or app. If you look at the docs for the app version of chrome.runtime.sendMessage, you'll see that it is identical to the extension version.