Why wouldn't a 1px CSS line, shown with 3 pixels on iPhone 6+ per, downsampled to HD (per the article) still look OK?
Here's the web page with the 1 pixel line: http://jsfiddle.net/k0xep8pL/embedded/result/
On my PC, the line is just a single pixel. On my Nexus 5 -- as you can see -- it's actually three lines, which are (from top to bottom): grey (RGB=134), black (RGB=0), grey (RGB=156).
The impression I get, when looking at it on my phone, is a solid black line. Bear in mind, that the width of the display is 61 millimeters, which means that these three pixels take up only 0.17 millimeters on the screen.