From china border (Lang Son, Ha Giang...) to DMZ is more than 700km
He say he all cycled, that was not easy and it sure took 15 - 30 days.
Anyway, that was possible.
As you see, The Vietnamese Demilitarized Zone ran along the Ben Hai River for much of its length, So to cross the river was not easy.
But i think that was possible for individual.