They previously used openSUSE and then Fedora. But both have companies behind them, where-as NixOS does not. And with NixOS they are more sovereign.
As a normal NixOS user, I know how to do this. It would be quite some effort, but not especially difficult. Additionally, for a government they have the servers to set up a build farm.
Why did those not work out? Why do they think things will be different with NixOS?