I think the devil is in the details of how iOS and watchOS interact. Instead of sending and receiving texts from iOS, watchOS is running messages and only sharing data. I imagine this pattern works a large amount of the time in Apple's benefit. They make a notification API available for anyone to use, they just choose not to do it. They also make it so the watch can use the phone's data, and this they do use, but also it can use WiFi or Cellular on its own.