I went pretty nuts with theories about whose level it was, but I never said "I KNOW IT'S THIS LEVEL!!!" or anything.
I just figured there was too much detail to be just some random level, what with the valley below, the crystal holding the level aloft, the moon, the sundial, the day night cycle, the platforms with the yellow borders and that strange blue pattern on the ground.
By comparison, look at Melee's "battlefield" level: bland as hell, hence why I figured it HAD to be someone's stage. Even Pap figured it was a Kirby level.