You're right of course, but if you take that to the extreme, styling a clickable action with a coloured square is skeumorphism too (it's still a button). The way I see it, the concept of a button has long since tracended into a metaphor in the visual language of UI. Where it used to mimic the physical world, it has since been reduced to a symbol. I think that most people would say that the beveling effect is part of this symbol. If that is the case, then Metro tries to redefine an established symbol. This isn't unique per se - languages evolve all the time, but it does explain why a lot of people might be confused of it.