The estimate sounds suspiciously similar to just the data storage component of DynamoDB. 1.7PB of data and indexes is about $5.1m/year in DynamoDB storage at list.
Supporting that, Uber’s blog post linked from the article mentions cost savings as a benefit from going from three systems to one, and doesn’t really mention any dollar figure afaict.
https://www.uber.com/en-AU/blog/migrating-from-dynamodb-to-l...