Wow. That's the strangest thing. How are they able to make everything legible???
TBH, I have no idea, I was very enjoying to play around the resolution on at that time. And Apple took that away in short period of time.
My understanding is that everything was done at the OS level. So the OS…
1) allow you to choose a higher resolution, even though the cable / monitor do not support it.
2) call the GPU to render everything at that resolution
3) scale the image back to the resolution that the monitor can support and display it
Therefore, if made a screen capture, it will capture the pre-scaled (original resolution) screen from the VRAM.
I really like that idea. Even though may not be very useful in daily life, but I can use 1440P HiDPI to make my display at the "native" resolution, but screenshot shows the full 5k 1440P picture, etc.
At that moment, 4K was quite new to OSX, may be Apple was finding its own way to handle 4K (or 5k) and that's just a result of "bug".
This "super resolution" stuff only worked at the desktop level, but not system level. If I enter a game, the max resolution will revert back to the monitor's max.