Why Distinct Icon Outlines Help Users Scan Faster

I've seen this exact advice in the Tango icon theme guidelines several years ago: http://tango.freedesktop.org/Tango_Icon_Theme_Guidelines#Fre...

But i think it only applies for low-contrast backgrounds, like in these examples, for high-contrast backgrounds it doesn't necessarily make scanning harder. It depends on whether multiple distinctive colors are used for the icon itself. Icons without distinctive shapes often only use one colors for the icon itself and another color (or even multiple colors) for the background. I think this complicates scanning much more.

Also, removing unnecessary complexity from icons is almost always benefiting. So, yes, if possible, you should remove meaningless background shapes.

Then why are all the icons in the iOS interface the same shape? I wonder if this isn't a oversimplification.

yet another article about UI where the author presents a hypothesis as though it were the conclusion of research. does nobody actually do usability testing before writing?