I don't know why the author mentioned that Ruby allows any value to be null. Languages like Ruby and Javascript don't have static type checking, therefore their handling of null "not even wrong" compared to Haskell and OCaml which allow you to have the compiler eliminate null pointer exceptions at run time.

IIRC, some Java dialects re-introduce the idea of a variable that cannot be null, and it is possible to do fancy tricks with annotations to produce the same effect.