He's not saying the 0% price was a secret from his company, he's saying it was a secret to the general public.
I pay pretty close attention, and I thought the free apps of the day were paid at the 20% rate that Amazon warns they may pay you if they unilaterally discount your app.
Their standard policy concerns the unilateral decisions. This was a mutual agreement outside of the standard policy. The benefit to the app maker is a guaranteed free-app-of-the-day promotional spot, the benefit to Amazon is not having to pay the 20%. Seems like a (potentially) fair deal.
I pay pretty close attention, and I thought the free apps of the day were paid at the 20% rate that Amazon warns they may pay you if they unilaterally discount your app.