The Tip of the Iceberg
The Radiant Heaven Realmship has its own independent world, but it is not cut off from the outside world—just like any other ship in the sea with its interior walled off by the hull and bulkheads, albeit incomparable in size.
Just because Leon couldn't see the boundless sea and vast skies of Gaia, it didn't mean he isn't on Gaia—that is if the realm spirit didn't lie to him about its location.
That being said, Leon was not completely sure that he could reach Gaia's Core like the past two instances.
He might be sitting on earthen soil, but it wasn't the earthen soil of Gaia; it was the earthen soil of the Radiant Heaven Realmship's subspace and the Eternal Night Demon Empress's inner world.
'Well, there's only one way to find out,' Leon thought to himself.
Shortly after closing his eyes, Leon spread out his divine sense and sunk his consciousness into the earth.