Generally speaking, many countries stipulate in their nationality laws that citizens born in their own countries automatically acquire their own nationality. Some countries adopt a unique nationality system, that is, a person who chooses the nationality of country A cannot have the nationality of country B at the same time. China and the ancient capital Mongolia are the only nation-states.
The one mentioned by the landlord may have been born in Mongolia, or he chose Mongolian nationality, so it is impossible to have China nationality at the same time.