the planet the kids inhabit at the end of the game IS Caliborne and Calliope's planet, but after they leave it. So it's Calliope and Caliborne planet, then its empty, and THEN it's inhabited by players.

 Vriska captchalogued the Earth then the kids fast forwarded it in time until it wasn’t flooded anymore brought it to Universe C, made a lot of baby everything then went to the future again. (most of this is seen in Act 7)

Far off in the future the Calis hatch and live there and Caliborn takes the planet with him into his session with it later turning into the Land of Colours and Mayhem.