In practice there's a big difference in thought process and workflow between drawing freehand on paper and flipping through a UI library with a drag and drop interface. I think this is true even if your task is just composing from the same well defined set of components.

If they can actually pull this off it I can see it being very useful. And if it works really well in conjunction with other experiments currently ongoing, I can imagine it even affecting the way teams are structured and how a designer's day to day work goes.