Earlier quoted context omitted.
Well now you're getting into statically vs dynamically typed, which doesn't apply to this conversation because both Ruby and Clojure are dynamically typed. But in a statically typed language like Scala or Haskell, you can use the typechecker to express proofs about program correctness. That is a form of "expressiveness" that doesn't really exist in Ruby or Clojure (unless you use core.typed).
There's nothing about dynamic typing that prevents declaring types right? Because in Ruby you while you can extend an instance, you can not taketh away Types from it. IOW, Ruby could use the Scala syntax I wrote, and still be just as Dynamic. You're already constrained by how you use that type within your method. You're just not expressing it concisely since you have to read the method to understand the implicit Type…
With Clojure you can actually compile your modules while developing them and discover certain classes of type errors ahead of time, so something like core.typed actually makes sense because you can run the typechecker over your code instead of having to run a separate test suite.
The main type error you always have to worry about in both languages, though, is accidentally calling functions/methods on nil at runtime. Of course that applies to null in Java too...