You can work it out if you read the article. It says the high infection/death rate estimates were based on no interventions and no change of behaviour i.e no voluntary social distancing and working from home. Looking at the reaction to the lockdown, from some sections of US society, it is pretty obvious that without the lockdowns enough people would have made no changes to their behaviour and would therefore contribute to higher infection levels and desths. Further, plenty of employers would not have allowed working from home and would have expected people to attend work or lose their jobs. So infected people, predominantly poorer people, would have had no choice but to work when ill and with the atrocious healthcare system would have been unable to access help even if they didn't have to work. So those huge numbers we saw estimates of at the beginning could easily have been achieved in the US, and possibly still could. Florida saw over 10k cases a day this week. This isn't tailing off or going away, it was slowed.

Vietnam had a quick response to the problem, but it also doesn't have the same level of international travellers passing through it so it is easier to contain. The US could have contained it but that means shutting 150 airports, as opposed to Vietnams 3 airports.

Vietnam gets about 45k visitors per day, in comparison the US/Mexico border crossing in California/Tijuana has 75k crossing per day. That's one border point and there is also the US/Canada border too.

So it is in part because Vietnam handled the pandemic better than the USA, but that was also more easily achieved due to the size and complexity of each nation.