Ah, it seems like a hack that some telcos are doing to get around the pairing problem.
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...