The author is wrong here, because that sentence exists to support the decision to not allow Adobe to interpose themselves between Apple's platform APIs and developers. Neither the Finder nor iTunes presented an impediment to improving the platform because neither of them interpose in any significant manner. That they have managed to bring MacOS X this far without touching them is evidence of this.
Regardless, Apple's own development is under Apple's own control. The current question is why they are not willing to be at the mercy of a vendor that is not only beyond Apple's control, but also has an established history of lagging. You may not like their decision, but it is prudent, consistent with their stated values, and consistent with their actions.
The author did not understand the function of this paragraph, and based his attack on a faulty interpretation.
I'm surprised that so many denizens of HN seem to be having trouble with the concept of a chain of dependencies, and how that makes the cases of iTunes and the Finder very different.