For posterity, the above is no longer correct. They updated their example to use 200 EC2 instances, which was most of the disparity, and it seems they do actual

For posterity, the above is no longer correct. They updated their example to use 200 EC2 instances, which was most of the disparity, and it seems they do actually use 1024 KB/MB in a KB/MB, while I had in my head from elsewhere in AWS that they don't do that...