Did you try searching for it? The first result on Google for "daringfireball.net flat design 2013" is https://daringfireball.net/2013/01/the_trend_against_skeuomo...

Tack on site:news.ycombinator.com, and you'll find the top comment too: https://news.ycombinator.com/item?id=5081618

Cheers.

Gruber does mention Metro:

The lack of skeuomorphic effects and almost extreme flatness of the “modern” (née Metro) Windows 8 interface is remarkably forward-thinking. It’s meant to look best on retina-caliber displays, not the sub-retina displays it debuted on (with Windows Phone 7.x) or the typical PC displays of today. That said, I think there’s a sterility to Metro that prevents it from being endearing. It epitomizes “flat” design, but I don’t think it’s great design.