I'm fairly certain that Moonlight Densetsu does exist in-world. In season one, Mamoru gave Usagi a music box that played an instrumental version of the song. I don't know how readily available the song might be, as the version with vocals might have been lost during the fall of the Moon Kingdom, depending on how Mamoru obtained it, but it did exist in-world at some point.
Edit:
I forgot to mention, I've noticed that a number of animes, especially older ones, sneak theme songs into the backround as music playing on TVs and radios, as it is one of the few pieces of music that the show already has the rights to, and usually doesn't need to pay extra to reuse. For example, I remember a scene in the second or third season of Ranma 1/2 where the season one theme song was playing on a TV. I can't recall, but I think they might even have included a miniaturized version of the visuals, though that part might be my imagination warping a years old memory.
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Stand between the Silver Crystal and the Golden Sea.
"Youngsters these days just have no appreciation for the magnificence of the legendary cucumber." --Krityan Elder, Tales of Vesperia.
Edit:
I forgot to mention, I've noticed that a number of animes, especially older ones, sneak theme songs into the backround as music playing on TVs and radios, as it is one of the few pieces of music that the show already has the rights to, and usually doesn't need to pay extra to reuse. For example, I remember a scene in the second or third season of Ranma 1/2 where the season one theme song was playing on a TV. I can't recall, but I think they might even have included a miniaturized version of the visuals, though that part might be my imagination warping a years old memory.
-----
Stand between the Silver Crystal and the Golden Sea.
"Youngsters these days just have no appreciation for the magnificence of the legendary cucumber." --Krityan Elder, Tales of Vesperia.