Why didn't China call the outer Mongols China people?

The premise of being called a China person is that this person belongs to the Chinese nation. The definition of the Chinese nation began after the founding of the Republic of China in 19 12. During the Republic of China, all officially recognized nationalities were collectively called the Chinese nation, and only those belonging to the Chinese nation could be called China people. Mongolia was controlled by the Soviet Union during the Republic of China and became independent. Actually, it doesn't belong to China anymore, so it's no longer called China or China.