Being able to briefly untether your watch is a FAR cry from it being “standalone”!
https://support.apple.com/en-us/HT204641
The instructions for updating it begin with “make sure your iPhone is on the latest version.
Standalone would imply that you could own and use an Apple Watch, WITHOUT owning/using an iPhone... which you simply cannot.
It is my understanding that the Apple Watch doesn’t have its own # registered on a network or anything, rather it does a fancy “cloning” of your iPhone SIM & passes the calls/messages/notifications sent to that device... so, if you canceled service on your iPhone, your Apple Watch would not do that anymore.
That is, by definition, a companion device (with limited autonomy), again... NOT a standalone device.