Thank you for making this point. As I read that claim, I simply laughed. Certainly Metro is the most recognizable name for a design motif characterized as "flat," and then as you've pointed out Google's Holo motif is a close second.

iOS is many things, but the forerunner of minimalist or flat design? Hardly.

To an extent, I agree with some replies that say, "Well, nevermind, that wasn't really central to the point being made." Yes, but it would be like me saying Java is at the forefront of first-class functions because Java 8 is doing lambdas and then going on to talk about first-class functions.