Got it. I was using the account id with an application key.
A more careful reading of the above linked page shows that:
0_1542076748186_e6061550-5167-4def-b9b5-5b89e859f408-image.png
So that was the disconnect.
Application Key requires Application ID.
Account ID requires Master Application Key.
When the examples clearly stated ACCOUNT_ID and APPLICATION_KEY
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