Why is it said that Koreans originated from Changbai Mountain?

In Korean textbooks, the area of ??Korea (Goryeo) is like this (see the picture). They call Changbai Mountain Paektu Mountain. Here is some information:

Recently in Changchun The Korean athlete who won the runner-up in the women's 3000-meter short track speed skating relay final of the Asian Winter Games put up the following slogan (picture) on the podium, which read: "Baektu Mountain (referring to Changbai Mountain) is our land"? 1. The Koreans applied to UNESCO for the "Dragon Boat Festival" and succeeded. ? 2. Koreans claim in textbooks that Korean ancestors "nurtured the Yellow River Civilization." ? 3. The Koreans took over the Central Plains and established the Shang Dynasty. ? 4. Korean ancestors invented "movable type printing". ? 5. The historical territory of Koreans includes today’s Outer Mongolia and Siberia (Goguryeo); starting from Shanghai in the east, Sichuan in the west, the Central Plains in the north, and Hainan Island and Taiwan (Silla) in the south; starting from Japan in the east and Xinjiang (Hundreds of Nations) in the west. Ji); west of Xichuan and south to Vietnam was the Tang Dynasty. ? 6. The medical ancestor of Koreans is "Shen Nong", and Chinese medicine was invented by Koreans and is called Korean medicine. ? 7. The Korean medical manual is "Compendium of Materia Medica", and Li Shizhen is Korean. ? 8. The ancestors of Koreans include Lao Tzu, and Korea is the origin of Taoism. ? 9. The next target of Korean obscenity---Confucius. Confucius was about to become Korean. ? 10. Koreans say that the Chinese understanding of "Seoul" is "the city of Han people", so they changed it to "Seoul". ? 11. Why are Korean women so beautiful? Because "Xi Shi" is their ancestor. ? 12. Koreans have recently made major archaeological discoveries: Oracle was invented by Koreans. ? 13. Koreans were the teachers of Dayu. ? 14. Korean textbooks insist that Manchuria, most of Northeast China, and most of the Shandong Peninsula are the inherent territories of Korea in history. ? 15. During the World Cup, the President of South Korea claimed that South Korea has a history of more than 5,000 years and has a long history. ? The above is the education that South Korea provides to Korean young people. Look at our China. The history textbooks have been revised. There are no longer important contents such as dynasty changes. They are all about the Yellow River civilization, artistic development, etc. Maybe after many years, our Chinese nation will really become As a nation without history, we still don’t know where we came from and where we will go.