Neither the data nor the article make any connection between USING Clojure and hourly wages.
The connection is between LEARNING Clojure and high wages.
This is commonly known as the Python paradox, cited in the article. Learning currently esoteric languages signals a motivated self learner passionate about his craft. Listing Clojure in a job listing signals a company doing work that would be interesting to a motivated self learner passionate about her craft.