It was the editor that was making shit up. You can read about this even on Wikipedia https://en.m.wikipedia.org/wiki/The_Black_Book_of_Communism [In particular, Margolin, who authored the book's chapter on Vietnam, stated that "he has never mentioned a million deaths in Vietnam"]
Huh I recall reading the wiki page about this sometime in the past decade and it presented the academic consensus that the BBofC was polemical bullshit and now the wiki page is like “controversial yet thought provoking”