This makes it sound as if macOS upscales a 4K render when displaying to (for example) 5K monitors, but on a 5K monitor everything is ultimately rendered at a full physical resolution of 5120x2880. But in the Displays Preference Pane, the logical resolution is set by default to 2560x1440 (2:1). One can choose a logical resolution of 5120x2880 (1:1), but I can't imagine anyone working like that.