Actually, now that I think about it, it makes sense. After all, even though the Cold Catastrophe had been devastating for a while, the Imperial Court still maintained strong control over China.
Although there were periods when the Imperial Court lost contact with various regions, these disruptions didn't last long, as the Court soon found new ways to restore communication with the rest of China.
Before the Catastrophe, there might have been people hiding in the deep forests beyond the Court's reach, but as disasters continued to strike, the wilderness became a forbidden zone for humans. Even someone like Dong Tianyu, who had prepared early to face the Cold Catastrophe, eventually moved to live in the city for a while.