I think his point is that it isn't Cocoa itself which makes the new version faster and more extensible: It's the rewriting process. The new codebase has presumably been better structured to be faster and more extensible.
As John Gruber has said "Cocoa is just an API. It is not some sort of magic technology where you just sprinkle a ton of square brackets in your source code and you instantly get a better UI." (http://daringfireball.net/linked/2008/10/30/cocoa-finder)
I agree 100%, but a well written cocoa app is much easier to achieve than a carbon app (one reason why the Finder has been screwed up all these years).
But I agree, a rewrite with performance and accessibility in mind is what makes it faster, not cocoa itself (should've reworded that).
As John Gruber has said "Cocoa is just an API. It is not some sort of magic technology where you just sprinkle a ton of square brackets in your source code and you instantly get a better UI." (http://daringfireball.net/linked/2008/10/30/cocoa-finder)