Scylla Enterprise [0] [1], which is a Cassandra-compatible NoSQL database, though I think that they were probably using the DynamoDB API shim to talk to it. The non-enterprise version is open source, but fewer features.

[0] https://www.scylladb.com/product/scylla-enterprise/

[1] https://www.aljazeera.com/economy/2021/1/14/it-could-be-neve...