[Kind of off-topic] I'm guessing you mean by 'than Vietnam', 'than Americans died in the Vietnam war"? (Well I think so. Apologies if not.) That's a pretty macabre unit. Can you imagine how a Vietnamese person would feel reading that? Maybe in Vietnam they say 'More people die from [something] every year than America' - meaning what they call the American War[0]. Wouldn't that sound super-weird. That's something like how what you said sounds to me. Only for them it would mean 1-3 million.
[0] or the Resistance War Against America.
edit: Downvoter, care to explain how I can improve my comment? Thanks.
I'm downvoting because you are getting offended for others (I'm assuming you are not Vietnamese). Avoiding common expressions because you are trying to guess that someone might be offended is going too far in caring and politeness.
ps. You could also use car deaths per year (somewhat similar number) but then anyone who has lost relatives in car accidents might get offended by using these macabre numbers.
They are just using synecdoche. From the context of the comment and the shared culture of the audience people can understand what they are talking about. It's no different than how people in America know that if you talk about changes in government policy after "nine eleven" you mean the terrorist attacks not the sports car.
Imagine you're sitting in Canada and say "The opiod epidemic kills more people annually than WW II", and then when confronted, you say "Of course I mean Canadian lives in WW II!"
Americans may only see 55000 deaths in Vietnam. The rest of the world (and HN is international in audience) sees a lot more.
Possibly a better way to put it is to simply point out that the war caused something like 1.3-4.2 million deaths.
A similar bit of misfocus shows up in comments about the Holocaust, which technically refers to the 6 million Jewish dead rather than the estimated 17 million total.
[Kind of off-topic] I'm guessing you mean by 'than Vietnam', 'than Americans died in the Vietnam war"? (Well I think so. Apologies if not.) That's a pretty macabre unit. Can you imagine how a Vietnamese person would feel reading that? Maybe in Vietnam they say 'More people die from [something] every year than America' - meaning what they call the American War[0]. Wouldn't that sound super-weird. That's something like how what you said sounds to me. Only for them it would mean 1-3 million.
[0] or the Resistance War Against America.
edit: Downvoter, care to explain how I can improve my comment? Thanks.