Hey, I thought Google invented flat! /s
Seriously though, Windows phones sucked for many reasons (including lack of exposed features in the OS, especially in first versions, and of course lack of apps in general), but UX wasn't one of them. I guess they failed in releasing too much of an MVP, and then not following through properly. I continue to claim that Microsoft had a great UX vision for mobile, and that they're the only one with an OS that can turn tablets from consumption devices into productive tools.
it was somewhat annoying to watch the community go religious on flat design when "Apple invented it"