Yes, it is a new thing to be able to set up somebody else’s watch from one’s phone. But, you have always been able to set up your Apple Watch from your iPhone.
Guys, I hate to say it but you seem to be missing the point of the cellular version of the Apple Watch and taking the thread in a different direction. Why would Apple even bother to offer a cellular version if it had to be tethered to the phone or the same cell tower that the phone is in? And Apple has been offering cellular versions of the watch for a very long time.