The People's Republic of China did send extensive aid to the Viet-Minh, typically in the form of artillery pieces, ammunition, and ant-aircraft guns. Some of these, the anit-aircraft guns for example, came attached with Chinese military personnel who trained/operated them for the Viet Minh. China also sent military advisors (e.g. engineering specialists) to the Viet-Minh, who severely lacked in technical expertise.

Over the course of the war the total number of Chinese military personnel in Vietnam supporting the Viet-Minh overall effort reached 320,000. But they were not volunteers and they were not all serving in the Battle of Dien Bien Phu. If that were true they would outnumber the Viet-Minh owns troop concentration 3-1.

The major Chinese contribution was the provision and advising of the Viet-Minh anti-aircraft guns based around the valley. Like all besieged fortresses, Dien Bien Phu lived and died by its supply line; constant flights of cargo planes delivering men and material. The 37mm FlaK emplacements allowed the Viet-Minh to threaten this line directly. Although he may be wrong about the numbers, your teacher is not incorrect in citing the presence of Chinese advisors and equipment as critical to the Viet Minh effort and a major contributor to their military success.

Fall, B. (2002). Hell in a Very Small Place. De Capo Press