More accurately: Why I develop this specific class of applications for the desktop rather than the web. This is basically what the author is talking about despite the generalization regarding desktop apps; if he's going to stick with that he should have gone with "desktop" instead of "mac."
You seem to have missed the whole "Features" section. Here's a list of what he said he gains "for free" by developing for Mac (of course, other environments might have similar stuff, but not exactly like OS X's): Undo/Redo with Core Data, PDF export (and "import" into current documents), Multi-touch zoom.
He was being very careful to not overstate his claims, or even to appear to be overstating his claims. Considering how people in forums typically respond, I understand why.
