They are expensive purely because they are a smaller pool. I doubt they are automatically more capable. I did work with many and now write clonjre myself. Doesn't make me, or them automatically better devs imo!
So of course that is interesting, probably says some important things, and moves the average salaries up as well. Everything points to "Clojure tends to attract some subset of better devs" rather than it making them better.
There are clearly also effects from the small pool of available candidates, the high output of experienced Clojure devs, etc but generally being in a smaller ecosystem doesn't drive prices up.