I think this is exactly what the article is suggesting.
quoting from the article
# In config/dev.exs
config :my_app, :twitter_api, MyApp.Twitter.Sandbox
# In config/test.exs
config :my_app, :twitter_api, MyApp.Twitter.InMemory
# In config/prod.exs
config :my_app, :twitter_api, MyApp.Twitter.HTTPClient