First of all, he really was still in the Heavenly Court.
No, more accurately, he was indeed still in the Primordial World—just in its outer reaches, beyond the Three Realms.
To be precise, this place should be the Outer Side of the Primordial World that Venerable Pangu had created when he split heaven and earth, centered around the Primordial World and carved into four great star regions.
These four great star regions, together with the Primordial World, made up the complete Primordial World of heaven and earth.
And the place he was now was one of those four great star regions, the South Heaven Star Region. It was called that because of the South Heaven Gate.
That's right—the gate the Great Sage in Journey to the West loved darting through most.
Originally, that thing's sole purpose had been to let people enter the Heavenly Court from the mortal realm.
But from time to time, heretics and fiends would emerge from beyond the Three Realms, each time stirring up no small amount of trouble.
This incurred the Haotian Emperor's wrath. To make it easier to govern the outside world—especially to block those heretics and fiends—His Majesty directly used the Heavenly Court's four great gates of east, west, south, and north to slice the outside world into four with a single chop, then used those four gates to contain the four great star regions.
From then on, if living beings from the outside world wanted to enter the Primordial World, they could only pass through these four gates.
Don't ask Li Su how the Heavenly Emperor had done it. Four gates had somehow sealed away a vast, boundless universe in reverse.
Ask, and the answer was: he didn't know!
Nonsense. Even the old monster who had supplied him with the information didn't