> Remember that the NodeJS runtime guarantees the setting of no environment variables. The NODE_ENV variable is simply a convention that was started by express.

You're right that the runtime itself doesn't guarantee any env vars. I'm not sure who used it first, but npm also sets NODE_ENV (and some other env vars I think) in its different lifecycles, depending on if you provide the --production/--no-production flag when running it. But not sure if that came after or before express started using it.