"most people wonder if the moon is part of the sky or if it's just a decoration added later" This line retains the themes of out-of-our-world observations of the sun and skies from the previous verses upon titularly flying around. It also retains a merit of scientific accuracy given as you've said in your own words: ''holy sh*t we just got hit with another ball of flaming rocks- and it kind of made a mess. which is 🎶now the moon🎶.' In essence this line maybe asks the question 'is the moon really real?' And while being musically a melodic 'echo' of 'sun is not very bright'
i can tell you that the musical/lyrical echo is the primary reason for this line's existence. i guess it's that part of the second verse where it's time to be mildly responsible in the songwriting. it echoes the 'some people wonder', and instead of "sun" it's "moon", which felt like the standard verse 1 to 2 transferrence. and the rest of the line is just the best option i could come up with for finishing it. it also seems to generally play a 3rd above the first verse's line, (not to mention an actual harmony added above it here) which playes into the mention of 'decoration', and it is 'later' in 2 ways actually, because the this verse's line comes later than the first, but, also because the vocal singing the higher harmony here came later than the lead