Author claims frustration with Django's settings, only to re-implement it almost exactly. How is ENV=production any different from DJANGO_SETTINGS_MODULE=myproject.settings.production?

Sure, by default Django may create a single settings.py, but common practice is to split that into a settings package, containing a base settings module for common settings, and other files for different scenarios, say "development" and "production". Either / all of them can load secrets etc. from the environment, so the "production" settings file is probably better thought of as "deployment" settings, when supporting multiple deployed environments e.g. staging and production.