DynamoDB is different from their published paper, which is mostly about designing a highly available key/value system that can be run on top of any other datastore. The paper does mention MySQL as a storage option.

The recent MySQL info comes from this 2016 thread on DynamoDB storing empty strings: https://news.ycombinator.com/item?id=13170746

Confirmed by this comment specifically: https://news.ycombinator.com/item?id=13173927

The latest 2018 ReInvent deep dive on DynamoDB doesn't reveal anything but still fits if mysql is powering the storage nodes: https://www.youtube.com/watch?v=yvBR71D0nAQ