My understanding is that it was perhaps second behind one in Baekje (so, still Korea). Are there other observatories that can be classified as such?
As for the distinction between astronomy and astrology, there really wasn't one at the time. Astrology led to Astronomy, but without high-quality lenses made out of high-quality Venetian sand, there wouldn't be real Astronomy. Astrology was math. It helped with knowing the seasons, celestial navigation, predicting eclipses and other omens. One of the smartest tech additions for Civ VI.
Something the Chinese had been doing for a long time before the Koreans had. I wonder where the Koreans got the idea from?

As for Civ VI, yes, it is a nice distinction.