When looking at the actual codebases inside, you are right is that Cocoa doesn't build on top of Carbon the same way that Powerplant builds on top of the Mac Toolbox. But, from the point of view of a developer looking at the APIs, especially a new developer to the platform, I think that looking at them as peers creates the Carbon vs. Cocoa discussions that we see. Looking at them as building on each other is a reinforcement of the view that object-oriented technologies build on procedural ones.
As you help to illustrate when talking about how both APIs utilize Quartz, CoreFoundation, etc -- it is actually simplistic to look at the APIs either as peers or building on each other, but for better or worse, we sometimes need such simplistic views even though no one simple view is an accurate reflection of the situation. And the primary myth that I am trying to debunk in the article is that either Cocoa or Carbon are the one true API for OS X.