That applies if you use bash. If you use another shell, you'll need to use that shell's methods to set environment variables in the config(s). And probably still need to set them for bash so that bash scripts get the correct env vars.

NixOS & home-manager do have functions that can set environment variables across all shells enabled via either of those systems. So I've got `programs.bash.enable = true;` && `programs.fish.enable = true;` && `home.sessionVariables = { EDITOR = "nvim"; };` in my home-manager config for example. A GUI editor for that would certainly be possible.