This is a completely meaningless article if they don't provide information about their technical stack, which AWS services they used to use, what TPS they are hitting, what storage size they're using, etc.
The story will be different for every business because every business has different needs.
Given the answer to "How much did migration and ongoing ops really cost?" it seems like they had an incredibly simple infrastructure on AWS, and it was really easy to move out. If you use a wider-range of services the cost savings are much more likely to cancel themselves.
TFA begins with a link to the original article with those details.
If you called "We used EKS" details, then yeah they provide those details.
Assuming this is indeed all they used, this was admittedly nonsense, they were essentially using cloud-based bare-metal.