My theory:
There is light, darkness, and nothing. Nothing is simply a void that connects the light and the dark. Every world in the light has a dark counterpart. I agree that this beach was the dark counterpart of Destiny Islands.
Here's where the theory gets original: I don't believe TT is in nothing. I believe it is in light. WTNW is its dark counterpart. Obviously, it should have been in the realm of darkness. My theory is that Org.13 was trying to bring it into light to unleash Nobodies into light. They had succeeded by the end of the game, but the plan backfired. Bringing it into light allowed the F'ING SWARM of Heartless to access it, hence the last Maleficent cutscene. So yeah...the only worlds that exist in Nothing are worlds that are created there. Castle Oblivion was such an example, so was WTNW before it was brought into light. The bottom line: only one time when Sora wasn't in the realm of light: when he entered KH and was at the dark beach.