ClojureScript does come with its own standard library just as you say. However, I don’t know if that’s CoffeeScript’s “biggest advantage,” althout it’s related. The way I would put it is that while the author claims that ClojureScript is “OO, the Good Parts,” CoffeeScript is JavaScript, The Good Parts.
Meaning, CoffeeScript programming is still very close in mental model to JavaScript programming. This is why it doesn’t require its own library, you are writing JavaScript and can use any abstraction or library you like.
Its syntax for OO programming is really sugar for using JavaScript’s built-in semantics in a legible way. Locally namespaced extensions are a wonderful idea, but they aren’t really JavaScript. For better or for worse, CoffeeScript eschews such deep departures from JavaScript.
That’s a disadvantage when you want something that’s a big improvement, but it’s an advantage when you are familiar with JavaScript and just want to get things done.