While I was also a bit incredulous about the neutron star thing, I thought about it a bit and they do seem to understand spatial magic a bit (or at least Luck and Filly do), so it's not too outlandish for them to have studied things related to this during all their research, even if their "technology" doesn't match up to ours. They (their "scientists") might use spells to detect interstellar radio waves, x-rays, etc.
Honestly, though, I would tend to agree that the author kind of just did whatever without thinking about it.