Re: At last - confirmation of the abundantly obvious
I agree with your point (good UX should not include unnecessary elements - even Microsoft state this in their UX design guidance) but the 'flat' UI problem is that it omits necessary elements. A completely blank screen with touchable areas doesn't need to show what is where if the user already knows, but even then the visual cues would help use it more efficiently.
FWIW no user intrinsically knows where to look for something without some initial visual cue. Although users can learn or be trained to make those assumptions, and a consistent UI will generally help increase usability, an assumption of intrinsic knowledge isn't a useful principle for designing generalised user interfaces (although understanding acquired knowledge is useful when designing modifications to UIs).
Also this is why we test usability design with users who lack familiarity with the prototype specifically to identify how effectively they will achieve the working knowledge to complete tasks.