I think they're loosely based on the Xiong Nu, who were the nomadic people that inspired the Great Wall of China, and may have been related to the Huns that later caused the Romans so many problems. The Mongols were almost a thousand years later.

But whether they're Xiong Nu, Hun, Mongols or Manchu, they're all get lumped together when discussing invaders from the steppes of Central Asia. They probably drew them like the Mongols because we have many examples of their armor and weapons, but not many drawings of what the Huns looked like. No one even really knows what Attila looked like.