My guess is that this is on-demand DynamoDB wrapped with a Cassandra-compatible frontend. Just based on the pricing and stated characteristics. It would be really hard to provide on-demand capacity for compute and storage using anything remotely similar to off-the-shelf Cassandra. The pricing looks like DynamoDB on-demand + a bit extra to cover the cost of operating the frontend.
EDIT: confirmed by an AWS employee here https://twitter.com/_msw_/status/1201924979647905792