Audible monthly subscription and purchases reimburse under Amex Platinum Streaming Credit

I am currently subscribed to an Audible win-back promotion originally seen on Amazon for $8.95 for the first 3 months. As expected this monthly charge credits under the new Amex Platinum Streaming Subscription credit, however it seems my purchase of the Audible audiobook upgrade for a owned Kindle books was also eligible for this credit.

Save your credits and spend cash for Audible books when the price of the title is less than your price for credits ($8.25 for $99/annual people)

• Discount: Amex is offering a $20 monthly credit for specific US streaming subscriptions. Audible Monthly Subscription and Audible purchases seem eligible
• Payment: Amex Personal Platinum Card

Estimated Total: $8.95 (Audible Subscription) + 7.49 (Audible Purchase) – 16.44 (Amex Streaming Subscription Credit) = Free after credits

Do test buys before a major purchase