You are right to be sceptical, and I think you're right about throughput: they mentioned that all tweets are ~22MB/sec. My hard disk writes 345MB/sec. A modern multicore processor should probably tear through that datastream.
However, you should realize that in a decent sized organization there are usually different teams working on different things that are evolving at different rates. You have to manage these people, systems and their changes over time. After awhile, efficiency drops in priority versus issues such as manageability and security... often due to separation of concerns requirements.
Beyond the build and maintenance issues, a serious challenge for real time services is high availability (HA). Tolerance for hardware/software/network/human failings must be built in. That also throws efficiency out the window. It's cheaper to treat cheap, easily replaceable machines as a service-oriented cluster than to build two high performance machines and properly test the full range of potential failover scenarios.