That /unsaved/{id} path with a unique server-assigned identifier means your diff content was transmitted to and stored on their servers.

Not necessarily. URLs can be changed client-side, within the browser, through JavaScript. The fact that the URL changed to unsaved alone is no proof. It could very well be browser-local, labeled unsaved and held in session store for example, ready to be saved.

With the other indications, you can of course make a guess and/or consider it a strong indication.

It should be pretty obvious/observable when observing interaction and network requests within the browser. A network request with the content as body would be much better proof.