I f****** hated my nestI have a pair of Nest thermostats in my home. They complain when anything is wrong: wifi disconnected, power isn't reliable on the c wire, etc.
It'll probably work without internet connection. But you won't gain any of the benefits of having smart devices - learning, optimizing to your schedule; remote controls and alerts, etc.
I've been talking with a colleague about trying to develop competitive devices that don't require phoning home, but it's a huge initial lift, and most of the market has demonstrated that it doesn't care enough about the difference.
There are optional tools to connect to your utility. I have no info about any back doors to be able to either confirm or deny.
I'm sure he's literally correct.
At that point, you would be better served by traditional programmables, which are still available and can be had for free or near to it through mass save.
Biggest pos i ever had