Note that this setup only works when your network consists of only machines that use the None backend (basically NixOS machines not provisioned with nixops). Any other nixops deployment would have problems, as the state is needed to retrieve data from the various API’s.
Thanks, @rbvermaa! I added a note about that to the post.
Nice! That approach should work well with GitHub - tweag/terraform-provider-nixos: Terraform provider for NixOS and NixOps where all the other cloud state is managed by terraform.