They aren't automatically more capable, it's just a well known and frequently noticed fact that Clojure tends to attract more senior developers, and the community is very senior dev heavy compared to other language communities.
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.
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.