You keep saying this, and it's just not true. The extreme skeuomorphism in the older iOS versions was just as maligned as flat design is today. I wouldn't mind adding some more shading to buttons to make them more easily identifiable, but let's leave the cheesy leather textures in the Calendar app dead and buried, please.
I'm not a fan of the Windows 95-era design either. As an example that comes to mind, traditional menus have atrocious usability. Ribbon-like interfaces that feature actual buttons you can click or tap on, with more important buttons larger and/or highlighted, have been a huge improvement over trying to wade through a sea of identical-looking pull-downs full of text. It's important to remember that the move away from pull-down menus was informed by a good deal of end-user research, a lot more than this article.
Regarding iOS, I'd like to see this study try modern UI fonts like San Francisco. There's a reason why Apple, Google, and Microsoft invested in new fonts, and that's because faces such as Helvetica (which, by the way, was used in iOS 6) that weren't designed for screens are harder to read.