False. We're not just talking about Closure. We're talking about Google Closure. They support millions of users with their apps and maintain cross-browser compatibility. Google is doing that huge block of work for ClojureScript in addition to whole-program optimization.
Google Closure is what I'm talking about too. However, it's important to know _which_ Google Closure, as there are a number of components. It seems that the component most used in ClojureScript is the JavaScript optimizer, which is most likely just run as a post-processing step.
Of course, ClojureScript could also be using the Closure Library within the code, which would integrate Closure deeper into the system.
