I think that the statement carries some amount of validity. Yet, I think that the problem here is that the statement presumes that French knowledge and understanding of Vietnam happens post- Dien Bien Phu. The French obviously did not "know much about Vietnam" before Dien Bien Phu. Had the French "knew Vietnam," they would not have been surprised at the resourcefulness and brutal efficiency of their adversary. Had they "known Vietnam," they would have understood the destructive effect of the monsoon on their operations and would have not allowed the airfields to be burned, removing help from the air as an option. The French did "know" Vietnam after Dien Bien Phu. The American experience of Tet featured the same amount of surprise and shocking efficiency. The Americans at Tet were similar to the French at Dien Bien Phu. Certainly, the turning of public opinion against the war in Vietnam and President Johnson's own acquiescence in refusing to seek another term are realizations that the French experienced in their own light after Dien Bien Phu. In this, the statement is valid. Nobody would be able to know Vietnam like the French. However, the French only got to "know Vietnam" after Dien Bien Phu. It is after this battle where full knowledge was evident.
In the 1954 battle of Dien Bien Phu, French forces drew Viet Minh forces into the hills of the northwestern part of Vietnam in an attempt to cut off Viet Minh supply lines from Laos. The French thought they could rely on being resupplied through aircraft and believed the Viet Minh had no anti-aircraft weapons. However, the Viet Minh did have anti-aircraft weaponry, and they placed their artillery in the mountains surrounding the French. After the Vietnamese forces occupied the highlands, they became invincible, and the French could no longer receive supplies by air. After the Viet Minh victory, the French decided to withdraw from French Indochina, and the Geneva Accords left Vietnam divided at the 17th parallel.
In 1968, during the Vietnam War, North Vietnamese and Vietcong forces launched an offensive on South Vietnamese cities during the Vietnamese New Year (called Tet). Though the offensive was eventually repulsed, images of fierce fighting turned western opinion increasingly against the war, and the United States eventually withdrew from Vietnam.
The statement "Nobody knows Vietnam like the French" has some validity if it refers to the idea that the French understood after Dien Bien Phu how difficult it was to win a war against Vietnamese forces. They understood that the Vietnamese were determined to fight for their independence—lessons the U.S. should have understood before becoming involved in the Vietnam War and repeating the mistakes of the French in Vietnam.