This black and unreachable boundary was unlike the boundary around his Cave Heaven. The boundary of his Cave Heaven was white and opaque, but it could be touched, and it was unbreakable for the inhabitants of his Cave Heaven.
After all, to destroy the boundary of his Cave Heaven is to destroy his Cave Heaven. So, anyone who wants to break the boundary of his Cave Heaven must have the power to actually destroy his Cave Heaven. This is why the inhabitants of his Cave Heaven don't have the strength to break the boundary of his Cave Heaven, even though they can touch it.
But in this situation, he can forget about destroying the wall because he can't make any sort of contact with it. As he tried to move closer to it, his position in space was changing. But this was only relative to the Sea of Worlds.
