The UI is still rendered to 5k. Fonts are rendered at 5120x2880 If you supply retina-compatible bitmap assets they are rendered to 5k. Lines are rendered at 5120x2880... UI elements may be positioned and sized using "normalised coordinates" of 2560x1440 (which isn't a big deal for laying out UI) but that doesn't make the resolution of a 5120x2880 "about the same res(olution)" (your words) as a 1440p screen... and in any case, that only affects UI layout, not the actual content rendered by retina-aware apps (I mean, just fire up a graphics app, draw a 1-pixel checkerboard and zoom to "actual pixels" size and you'll see 5k addressable pixels - although you'll get nose prints on the screen trying to make them out).
Maybe you do understand - but that's not reflected in what you posted, and other people clearly don't. The other person I was replying to literally said that the studio display renders at 2560x1440, which is simply not true - if you render at 2560x1440 you get a 2560x1440 image and the 5k display would look no better than a 2010 1440p cinema display. People would have noticed.
