A 5K display is strictly 5120x2880. A 5K2K display is not a 5K display. That’s why it’s called 5K2K, but not 5K. In other words it’s an almost 5K display that lacks the vertical pixels that would make it a true 5K.
That is wrong
5K = 5120 x 2880
5K2K = 5120 x 2160
It has nothing to do with „not a 5K display and that’s why we give it another name“. Nothing.
compared to a real retina screen
„Retina“ for screens (like „Retina Display“) is just marketing by Apple.
There is no official definition for it. Even Apple has no definition.