Greek was also the lingua franca of philosophy, mathematics, etc. in the Eastern Mediterranean for several centuries. If we are talking about astronomy, it is also e.g. common to call Ptolemy a “Greek” astronomer, even though he lived in the Roman province of Egypt.
I also feel the need to point out how hilariously pedantic this conversation is, and how much I enjoy that.
According to Wikipedia, it is speculated that Lucian’s native language may have been Syriac (Aramaic). But then all of his writing was done in Greek, mostly while he was living in Athens (after a lawyer/philosophy professor career traveling around the Roman empire). https://en.wikipedia.org/wiki/Lucian
When I hear "Ancient Greece" I generally think of Greece through the Hellenistic period. Though now that I've said that, Lucian would have lived through the Roman annexation of Greece.
Really, my point was Lucian lived at the more recent end of antiquity. Humanity's knowledge of the world around them was quite a bit different then than 500 years prior.