It's really quite laughable that, despite having battled with the Ratmen, Duanmu Huai and his companions, along with most people in this world, still believe that Ratmen don't exist.
The reason is actually quite simple.
Most people who have seen Ratmen are now dead.
As previously mentioned, the Ratmen's method of combat is unique. They don't immediately rush into battle, but instead, they first release a plague through masses of rats, significantly weakening the life force in a city. Then, when the city's resistance is basically shattered, the Ratmen swarm into the city through tunnels. By that time, facing the incomparable quantity of Ratmen, the diseased soldiers have nothing but death to look forward to.