If an alien makes it all the way to the moon with the tech to read the disc why wouldn't they hop on over to Earth where there would be much more data? When Carl Sagan did it it was much more evocative because Voyager will be traveling for thousands of years into deep space.
If you read the FAQ[1], it says:
> [aliens] Probably not … but well, who knows? No, the Lunar challenge is mainly about pioneer spirit, curiosity, and visions for humanity. So, with the symbolic act of leaving a snapshot of human history on the surface of the Moon, we are thinking more about future generations than aliens.