Yes, but that's a very different kind of system. AWS has to deal with a very large number of servers handling authentication for client that aren't trusted by

Yes, but that's a very different kind of system. AWS has to deal with a very large number of servers handling authentication for client that aren't trusted by the credential owner. HN has clients with credentials accessing a single server. It would be much easier to just store the serialized data in Memcached or Redis since the system is already centralized and use a token to look it up. Doing so requires less bandwidth and less messing with cryptography.