That's a non-factor
DPC actually renders in 4K, he just then downscales the images to 1080p for the game itself, but he keeps the original 4K ones for different special renders because it's usually just a series of several renders (i.e. the clone series, custom renders, wallpapers, etc.)
Now, you wonder why he doesn't also leave the in-game renders in their original 4K resolution for the end product? Well, check the size difference between the latest Sally renders in both resolutions
And now imagine the size of the game if all of those thousands of renders were in 4K and not Full HD