The author, Paul Biggar, was the founder of CircleCI which does use Clojure(Script). I don't know if he has written on why he chose not to use the same stack for Darklang.
The CV appears old. His recent position is listed as starting July 2010 to present, but looking at his LinkedIn we see his finished at Mozilla in 2011.
https://www.linkedin.com/in/paulbiggar/