Judging by this release, Amazon seems to acknowledge they cannot handle payments as well as Stripe (makes sense, companies focusing on one thing usually does that thing better than a company focusing on 100s of different things). Also, isn't the saying something like "outsource anything that isn't your core competence?" and I'm guessing Amazon is doing just that here.
This statement is what lead me to have the above perspective:
> “Stripe has been a trusted partner, helping accelerate our business at every turn,” said Max Bardon, vice president of payments, Amazon. “In particular, we value Stripe’s reliability. Even during peak days like Prime Day, Black Friday, and Cyber Monday, Stripe delivers industry-leading uptime. We appreciate Stripe’s relentless commitment to putting users first.”
Translated: Stripe has better reliability than the systems we've setup ourselves, especially during peak days.