The exact quote is:
- To receive SMS, MMS, or push notifications from third-party apps on your cellular Apple Watch, your paired iPhone must be powered on and connected to Wi-Fi or cellular, but it doesn't need to be nearby. You also need to be signed in to iMessage on your iPhone.
it does not say anything about voice calls. And it does not say anything about iMessage. It does not say anything about the dozens of other features the Apple Watch offers. It just talks about SMS/MMS
SMS/MMS is the old original cellphone TEXTing that piggybacks on top of the cellular voice network, using the 140 unused bytes in a cellular signaling packet. They charged outrageous prices per TEXT for something that cost them nothing, because they were sending these mostly empty signaling packets anyway.
If you have a Cellular Apple Watch, and access to cellular radio towers, and you have an Apple Watch contract with a cellular carrier, then you can make/receive phone calls, send/receive iMessages, have Walkie-Talkie conversations, stream music, get the weather, ask Siri for things, etc… all without any help from your iPhone.
But if you want to send an old school SMS TEXT or MMS picture to a user with an old flip phone, candy bar phone, or Android phone, you need your iPhone powered on, able to access the Internet (but can be on the other side of the Earth), so it can relay you SMS or MMS TEXT to that person. But you could also just call them if it was that important and your iPhone was out of action.