During the beginning of the Liberation war of china, the two army groups belonged to the Fourth field army of china were special troops in PLA,because they were fully consisted of koreans living in china. After 1949, Kim asked Mao to give the two army group to the north Korea, and Mao met the request of Kim. From then the commander of the two army groups is Kim not Mao.
So the key point is whether the two army groups should be consider the troops of the North korea or china. I think maybe we have different ideas about this.
If the two army groups are the troops of the north korea, obviousely you have drawn a wrong conclusion from a wrong premise.
By the way, after the korea war, officers of the two army groups were cleanout by Kim on order to cut down the influence from china.