What you are missing is somehow being obscured by mud in the water.
-The cMP will LIKELY be able to run 5K via a single DP cable(1.3) when those GPUs become available. Whether it will be able to do so with current GPUs using Dual DP cables (1.2) is up to Apple/Nvidia/AMD writing drivers.
-The nMP will most likely NEVER be able to run a 5K with a single cable as it is forever stuck in DP 1.2 mode courtesy of the non-upgradeable GPUs. (some pedantic nit-wit will doubtless point out that the GPUs CAN be upgraded since they are screwed in. I would respond that having nothing to upgrade to makes them non-upgradeable, screws, latches, or velcro notwithstanding) Whether it will be able to do so with current GPUs using Dual DP cables (1.2) is up to Apple/Nvidia/AMD writing drivers.
Thus, at least as far as 3rd party 5K displays, the cMP stands equal or better chances of running those displays. Admitting that the can has already been pushed toward "obsolete" by a product that Apple introduced has proven especially difficult for some.