Taking my own Civ 6 experience as at least somewhat representative, I assume many 4K players use the extra resolution to expand visible area rather than solely

Taking my own Civ 6 experience as at least somewhat representative, I assume many 4K players use the extra resolution to expand visible area rather than solely to increase graphical detail. And more visible terrain means more bits independently animated (presumably) under CPU control, leading to increased CPU usage in typical 4K scenarios. It's also possible that the "high", "medium", and "low" referenced in the article don't only refer to graphics, but also include important gameplay settings like map size and number of rival civilizations that can significantly affect average per-turn wall clock time required to process enemy AI on a given CPU, which IME is a considerably more important metric than framerate when it comes to the playability of any Civ game.