Well, MacRuby is a language on top of Objective-C. You can use Cocoa with it, but you should be able to use the Chameleon framework too, assuming it is GC-friendly (otherwise, adding GC support shouldn't be hard). His points don't make much sense to me.
Though, I think the difficulty of making a complex GUI in Cocoa shines in the OS X world. It's a lot harder to make a working UI, so you want to get the design right the first time, so you don't have to go back and re-do.