I think you're getting distracted by the simplistic implementation presented here. multimethods are plenty powerful outside of static types - see CLOS, Clojure.

That said multimethods are a halfway step to predicate dispatch (this is mentioned in the article comments). With predicate dispatch you can get efficient dispatch on any interesting predicate - not just types - for example even?, odd?, structural matching, etc.

I think dynamic languages could benefit a lot from predicate dispatch.