That's part of it but equally important is the idea of separating language / syntax changes, Objective-J, from the real goal of the library, Cappuccino. The other frameworks mentioned blend the two together, for example you can't get the JavaScript extensions implemented by Prototype without including all of Prototype.

But, with Objective-J you can write for example, some math library that has nothing to do with Cappuccino and not load Cappuccino.