I don't understand the division of government between Wei Yenwu and Lin Gray

Wei Yenwu oversees the legitimate government of Lungmen, under the authority granted to him by the Emperor of Yan. While he has a secret task force to take care of things outside of the system of law, he maintains the Lungmen Guard Department to maintain a facade of law and order.

But he also works together with Lin Gray, the Rat King who rules over the illegitimate underworld that defies Wei's laws.

The Rat King's only known law is like Sicilia's law in Siracusa: "Do not kill each other, do not kill civilians, do NOT make me come over there."

How can this contradiction function?

Was Lungmen so spacious, so populous, that after these two men took it back from Kaschei and the Empire of Ursus, Wei simply couldn't muster the man-power to assert law and order across the whole city?

And so Lin Gray volunteered to become the Rat King who would impose any kind of order over the lawless darkness of the city?