Depends on exactly what you mean by "mirror to retina devices" - if Affinity is drawing its view on one of these devices (screens?) then yes, but if it's another application that you're viewing them in, then you'll see no difference.
Basically we used to draw pixel documents as though they were points - a 100x100 pixel document used to take up 100x100 points on your display (which is what you'd get if you put it on a website) but now we put 1 pixel on each pixel of your display, so a retina display will now show images half the size it used to - as per Photoshop.