This means that the latest power-user laptop is more than capable of driving Dell’s UP2715K 27” 5K monitor (which you can see on the right) whenever it comes equipped with the optional AMD Radeon R9 M370X graphics chip. Prior to this, the most recent Mac Pro and iMac with Retina 5K display models had received certification for use with that monitor, and that Dell monitor is still maintaining an iron grip on the fact that this is the sole 5K monitor that is officially supported by Apple.
No idea on why Apple has not included target display mode for the Retina iMac to date, although one can speculate that bandwidth restrictions imposed by DisplayPort 1.2 could be a leading cause.