Not sure if Hiya is device specific, but it's a preinstalled Samsung System app on my S20 Ultra.
So you're right, Samsung Support is clueless again.
And you're right again, Hiya has nothing to do with your Carrier, Hiya may be tied to your CSC, but it's definitely a system app, which means that you can't uninstall it, only as your niece discovered, disable it.
I think that the tip off as to why they don't want to use Hiya, was the word "tracking". I get concerns about privacy, but seriously, if you've been on the Internet for any length of time, privacy is a unicorn.
Just be smart about what you do on the Internet.