That is nothing short of insane. Just because a few applications on iOS use a flat(ter) design, iOS is the leading edge of flat design? That makes no sense, when there's Windows Phone and Windows 8, two high-profile products from one of the largest technology companies in the world which have adopted this design style to its very extremes.
Heck, even Google has adopted this flat design to a great degree with Holo and, by lack of a better name, the "Google Now"-theme.
Yes, there is a trend towards this flat design, but if you're looking for its leading edge, iOS, and Apple as a whole, is about as far removed from said edge as possible.
I'm not sure if Gruber is intentionally dishonest, or if he actually believes what he's writing here. I'm not sure which is worse.