He actually touched on this towards the end of the presentation. To simplify, he states that Clojure is about data and Scala is about types. When accommodating dynamic data, it's easier to use Clojure because of the ubiquity of Maps as the primary data type and the nice handling if Null values via Clojure's Nil type. In Scala, dynamic data is dealt with using the Option Monad, and this adds layers of complication to your code.