You have a source on this claim? Unless they've managed the impossible, they're not immune inside. There are ALWAYS attack vectors unless you're using cups and strings.
I’ve talked to people who worked there. They take large data dumps from the internet and save them on their internal network. You feel like you’re on the internet, but you’re not. That’s how they keep ‘positive control’. I would imagine the site in Utah is for collecting Huge data dumps from the internet, then they can analyze them without being on the internet.