Edit - missed the edit window but the actual study goes on to say this:
"These findings also confirm that flat or flat-ish designs can work better in certain conditions than others. "
"Notice that those characteristics are also just good, basic UX design best practice: visual simplicity, external consistency, clear visual hierarchy, and contrast. In general, if you have an experienced UX team that cares about user research, you’ll do better with a flat design than other product teams that don’t. If your designs are already strong, any potential weakness introduced by flat design will be mitigated. If you’re conducting regular user research, any mistakes you make in implementing a flat UI will be identified and corrected."
So what we are now showing is that actually, this isn't a Flat vs 3D simple study. Which means that the Reg article we are discussing, is slanted differently to the conclusions actually put forward by the NN Group.