I heard same story told slightly differently - for services that process your requests when you go to amazon.com to buy stuff, you can expect engineering excellency. That would be dynamo, s3, ec2, networking (that whole side of AWS - hypervisors, networks, etc., I'd call engineering marvel).

But that would not be software they build "for someone else", like Redshift and Glue and Sagemaker and whole lot of other stuff. There's a whole lot of half-baked services in the ~200 they have.