As you point out, there are a variety of variables that could be tuned to eek out better performance here, and they could bring the two clouds closer. Our claim, of course, only applies to the benchmark configuration we tested with. That being said, with the size of machine we were restricting our testing to (16 vCPUS), no AWS machine claimed to offer more than 25G of throughput.
From my understanding of the test scenario, you are using a single TCP connection to run the throughput test, and hitting AWS' documented[1] throughput limit for a single flow: 10 Gbps if the two instances are in the same placement group and 5 Gbps otherwise. The reason some of the network-optimized instances were "slower" than the non-optimized ones is most likely a random draw of whether both instances in the test were physically close to each other (basically whether or not they are accidentally in placement groups).
To show the true throughput you would need to use multiple connections/flows, 5-10 would probably suffice. If the single flow test case was important then maybe you should have mentioned that AWS has a specific limitation around this. Personally I don't think a single flow test case is particularly realistic for a throughput test. Either way, how it is presented is pretty misleading.
1. "Single TCP flow is limited to 10 Gbps for instances in the same placement group and 5 Gbps between instances anywhere else." https://docs.aws.amazon.com/whitepapers/latest/ec2-networkin...