Yeah, I've noticed that too and even counted the pixels, the "squares" all seem to be 16x16 pixels. I would immediately know that it's an upscale if the squares were all 4x4, because if you upscale a 1080p image to 4K to get a 1:1 image with no aliasing you make one pixel four pixels. But yeah, I still don't know the answer to my original question unfortunately.