There's a somewhat throwaway line in there - "It’s as though Apple used the Yosemite wallpaper as the environment map for the new icons." - that made me think how it would be possible to find the highlight/shadow hue for any given wallpaper and apply it to the icon set. I imagine it would be technically easy to accomplish with vector graphics.
You could do it to raster graphics too with a gradient map, replace a greyscale gradient with a subtly colorised one. This would possible require a separate layer channel on the icon for the regions to be mapped.
