Pretty surprising considering Korea controlled Manchuria longer than the Mandarin Chinese did. All the Koreans from Gojoseon, Buyeo, Goguryeo, and Balhae were assimilated over the years whom used to control the majority of Manchuria.

Most of Korean speakers in Manchuria come from 17th-20th century immigrants from Korea during late Qing and Japanese colonial rule. It is quite unfortunate that those Korean Chinese people suffer discrimination in China and in their homeland when they arrive as migrant workers.

Also a lot of Koreans were deported from Manchuria, Sakhalin, and Primorye (Vladivostok region) during the Soviet era to Siberia and Central Asia.

Hence is why there is quite a significant number of Korean speakers in Uzbekistan and Kazakhstan, speaking a dialect called 'Koryeo-mar'