The PLA’s performance in the 1979 war was so bad, even Vietnamese commanders were surprised, according to some sources. This was a result of its reliance on Korean War–style infantry assault tactics, due to the operational inflexibility and stagnation of military thought in the PLA. The layout of the command structure, and the infrastructure that supported it, could not support maneuver warfare by smaller units of higher-quality forces.
https://nationalinterest.org/blog/reboot/whoops-how-vietnam-beat-china-1979-war-169687