If the 27" iMac could get above the retina curve on that graph, it still wouldn't be iPhone-like-retina unless it could get a PPI of 300 or more.
It would at the right distance. The whole point of the graph is to show:
a) Your phone and your computer isn't (and shouldn't be) at the same distance from your eyes.
b) The different distances allows a lower PPI to look just as clear.