Essentially when you supersample, the image is downsampled to the displays native input.
In the case of the pimax 4k & 8k the native input resolution is less than the panel's native resolution.
So say SS to 4k uhd 3840*2160 is downsampled to qhd 2560*1440. Then the scaling chip upscaled the image to the panel's native Resolution of 3840*2160.
Hope that explains it. Essentially the SS image should create a better quality image than a native qhd image (reducing the need to Anti alias).