Doesn't really surprise me seeing that in there. Nielsen is from way back in the usability community. You have to remember that it was considered revolutionary then to make buttons look 3D, like they stick out, and should be touched. Like they are real buttons. Gloss, shadows, etc.. User affordances. It's been backed up by heavy studies as well.

The modern flat movement, on the other hand, is more just graphics designers making something that looks good and doesn't work well. I've put users through user studies on converted Android apps and they have a hell of a lot harder time figuring out they are supposed to tap an understated flat icon with no label in the top corner, the new action bar pattern, instead of a big glossy shadowed button at the bottom like earlier designs.

I've brought these numbers to Googlers before at conferences, but they just shrug and say users will get used to it. So even Google admits the current flat designs are testing poorly and that they just hope numbers will improve as users learn to look for the flat hidden icons.