Their claim is: Erlang is good for building scalable distributed systems. Scala has actors too, but Erlang is more than actors (a lot more!). It's a language created to build distributed systems and highly scalable servers.

On the other hand, Scala is good for data crunching and mutable/immutable data processing. And you'd have a hard time dealing with mutable data on Erlang. As they are building a highly scalable data analytics system, it makes a lot of sense to couple the two languages. But I guess Akka is good enough to the other 98% of the use cases you may ever find. ;)