Not mentioned in that Wikipedia article, but the explanation for "why is there no ancient plastic around" is that the civilization could have pre-dated oil. Not enough time may have passed for significant amounts of the stuff to accumulate in the planet, so the civilization would have never developed a use for it. It would have been a rarity to them, not a foundational cornerstone of civilization.

It's a cool thought experiment, but also very cool to think about how much we lucked out not just to be on Earth, but to be on Earth at this point in time. Emerge a bit earlier and suddenly industrialization becomes exponentially harder.