Don’t know what to tell you. I am using this as a completely standalone plan: https://www.usmobile.com/apple-watch-cellular-plan (see Standalone Plan section)
From the page you linked:
> Your iPhone and Apple Watch will have different numbers and Apple IDs, but will need to be members of the same iCloud Family.
And from this Reddit discussion, it looks like texts won't sync:
https://www.reddit.com/r/USMobile/comments/1q8wpie/is_the_st...
How could SMS messages from different phone numbers sync? That’s the whole point: It is truly standalone. In my case there is no issue whatsoever because the person using the watch has no cell phone (preteen child).
For the majority of adults who want the seamless Apple Watch approach where their watch and phone act together contextually, this is not a solution but it works great as a true standalone “phone on your wrist”.