True, but one would hope that both sides in this case would be putting their best foot forward. Getting peak performance out of right sizing your DB is part of that discussion. I can't imagine AWS would put down "126 million QPS" if they COULD have provided a larger instance that could deliver "200 million QPS", right? We have to assume at some point that both sides are putting their best foot forward given the service.
The 126M QPS number was certainly parts of Amazon.com retail that powers Prime Day not all of DDB traffic. If we were to add up all of DDB's volume, it would be way higher. At least a magnitude if not more.
Large parts of AWS itself uses DDB - both control plane and data plane. For instance, every message sent to AWS IoT will internally translate to multiple calls to DDB (reads and writes) as the message flows through the different parts of the system. IoT itself is millions of RPS and that is just one small-ish AWS service.
