I have been using Dell P2415Q (24" 180ppi) for a few years side-by-side with a Retina MacBook, and the difference is visible: I can see pixels on 180ppi text if I lean forward, but not on the 220ppi. Not a deal breaker, of course, but the "retina" marketing pitch isn't lying.
BTW, LG 24UD58 mentioned in the article is also only 180ppi.
I am using the same, as I needed an external screen for my MacBook. I use it in scaled resolution for screen estate (2300 horizontal pixels), which works, but isn't as sharp as my iMac.
BTW, LG 24UD58 mentioned in the article is also only 180ppi.