Not to be combative, but I think what he means is this: Although it's perfectly easy to use JavaScript from Objective-J, the same is not true in reverse. ObjJ generates code that looks like this:
... where objj_msgSend is how the Objective-J "interpreter" does it's magic. If I wanted to use a library that was originally written in Objective-J, from JavaScript, I'd have a hell of a hard time calling it correctly, unless the library author took special care to make it JavaScript-compatible in the first place. The syntaxes may not collide, but the semantics certainly don't match up.
On the other hand, you'd never know that you were calling a CoffeeScript library from JS, unless you inspected the source, and vice-versa. In that sense, they're interoperable.
No not combative at all, what you say makes perfect sense. I suppose the confusion arose because he bundled objj with gwt, saying you'd need a totally different jquery, which is certainly not the case with objj.
On the other hand, you'd never know that you were calling a CoffeeScript library from JS, unless you inspected the source, and vice-versa. In that sense, they're interoperable.