I think it's more to highlight its strangeness, unknowableness—to us as well as Severian: "metals…that were green & blue in the same sense that copper is said to be red or silver white, colored metals so curiously wrought [they might have been] works of art or…strange machines" Yet Severian knows>

>machines from his time in the Citadel, the room in which they found the Revolutionary, etc. I always thought the fossil city (preserved BENEATH trees that had been petrified to stone; colors of its tiles so bright they seemed freshly-painted) was from a time in our distant future, his distant past