No it's not remotely 'insane', it's just mildly ignorant. As for 'entirely undermined', clearly the important takeaway from the article has nothing to do with which platform is at the leading edge.
And if you follow the thesis that "retina" quality displays are what's encouraging flat ui design. it's hard to get away from the fact that Apple has been leading the way on that front.
Gruber has been a very vocal critics of Apple's hard on for skeuomorphism for a long time. When he says the "timing of this trend, and the fact that iOS — and the iPhone in particular — is its leading edge, is not coincidental" he is most certainly not talking about the OS itself. This should be obvious because in the very next paragraph he says that "the whole default iOS look" is made up of "phony effects". But I guess if you're just here to get your HN rubber stamp upvotes by calling Gruber a one dimensional Apple shill you might have missed that.
(And I would agree that he overestimates the universal love for Letterpress, but basically all of the blogosphere was fawning over it. How much $$$ has it raked in compared to Carcassonne, which is almost a parody of skeuomorphism?)