Have you looked at the source code for that project? It doesn't use callables at all. the orig_commute function, the original commute method on class Ref, gets passed a monkey-patched ruby Proc object that implements the invoke method which takes one argument, a list, containing all of the original ruby arguments. Clojure then most likely calls invoke inside orig_commute through reflection. The end result: a bunch of hackery specific to Ruby and Clojure, something JVM-level closures would help eliminate.
Again, you cannot pass a ruby function expecting arguments to Clojure! What your link shows is that you can, however, send monkey-patched versions of ruby functions that implement clojure-specific calling conventions to clojure. :)