I failed a brain check, I've been googling those names for 30 minutes. I don't get the reference. What's the joke here? I would have just taken it as cleaver naming for the region.
Each one is a combination of 'Celestial Object' and 'synonym for deer'. The first two seem innocuous and fantastic enough, but they establish the pattern - letting the third be 'Starbucks', a real-life coffee shop chain.
Starbucks is a real coffee chain that exists in the real world. Moondeer and sunfawn follow the same naming scheme, but the players didn't realize that was what the DM was building to until the big reveal. It's...pun-adjacent.
I'm not clear on the details, but I know the constellations are made out of stars, I think planets like mars were thought to be major stars, and I'd think sayings like "the stars aligned" would have roots in astrology...
I will also nitpick and say that they said astrology terms, specifically - if astrology considers constellations to be important, and acknowledges they are made out of stars, I'd imagine stars would be part of the terminology. (Doubly so if I'm correct about astrology having (at least previously) a skewed view on what a star is!)
Constellations are symbols in astrology, but stars themselves are not. Sure, constellations are made of stars, but words are made of ink and yet I wouldn't say ink is a topic of literature.