I guess I don't understand where Erlang would fit in your (my) stack.
If you want to handle millions->billions of external client connections, you may want to be able to pack as many on to a machine as possible to reduce costs (the difference between paying for 5,000 machines vs 10,000 machines). If you want these to be secure connections (which I do), there is significant CPU overhead in encrypting and decrypting all network traffic. Even if you aren't doing encryption work, there is still significant CPU work in managing large numbers of sockets. Implementations may also vary significantly in the amount of memory required to support the connection, affecting costs and machine count.
I once got asked to spin off an ec2 instance from an image provided by the company I applied to, and then answer certain questions about the dataset on that thing. I had to write actual code to obtain the answers. All that had to happen within 24 hours. Then there was an on-site interview where they asked questions about stuff like JVM garbage collector fine-tuning and such. That was the best interview I had, ever.