If you only need to set an auth token to access private NPM modules, you only need to set the
NPM_TOKEN env variable.
You do not need to create a .npmrc file in your repo as the npm build process will detect the env var and do it for you. This will not help you if you also need to use a different registry, but my guess is most folks just need the token set.
I hope the netlify team will update the instructions and point this out! It would save a lot of folks some hassle.