Not really a rebuild, as it was just the Moon. But where did the debris go? Still in the same rough area/orbit? Maybe that rained down and caused destruction that the rebuilding is for? Did it affect the Moon's tidal effect much (not if it's in the same area maybe)?

Then the question of why they didn't come back.

Mistranslation? They're coming back probably, but the months might have been Earth years or more?

Something worse got them, and potentially knows now about us in some way (dark forest issue)?

It was a test to spark us in progress, either to ready us for something the aliens actually need help with, or to see if we're a danger at all to them with the right motivation (a bit of dark forest there too). Or maybe the only way a species can break through the Great Filter and not kill themselves is to be united by a common threat, and once past that point they survive much better, so they're nurturing in an odd way.

Something else entirely, where we think we're that important to come back to, but the aliens were distracted by far bigger things, not just another enemy but some other threat to them. Maybe a full tech/physics failure, i.e. like in Star Trek Discovery with warp drives, and what was terrible for warp-capable species doesn't affect us since we weren't at that level, so we find other methods and end up being dominate.