This article definitely helps for the new bees in sketch application.
I have a doubt with my understanding to this article. As said for example when we design with art-board of iPhone 6 at 640x1136 px as the base, I think the export asset option differs from what you have shown in screen shot. Because the base design itself is @2x size.
For example, if I'm creating a 50x50 icon in artboard size (iPhone 6 at 640x1136) my export asset should be 25x25@1x, 50x50@2x, 100x100@3x. To get this I had to set my export values similar to one that I have attached.
Correct me if I'm wrong. Even I have recently started using sketch tool.