I'm surprised by the author's hate towards DynamoDB. It's probably one of my favorite AWS Services. Great availability and no operational overhead. Cost was pretty minimal too each time I've used it, but you do need to spend some time architecting your data model up front, and that requires reading service docs and understanding it.
I built an app a few years ago and needed some sort of DB to store around 50 million records that had ~10k reads+writes per month with 1 index. It cost me something like $50 to load it up initially, and then something stupid like 10 cents/month to maintain.