Well 20% of NYC had COVID and we are currently seeing outbreaks in communities that have relaxed social distancing measures, so it seems obvious that her calculations are incorrect.
That’s part of the thesis of the article. Some populations mix more than others. In the mixing populations the threshold will be higher. In populations with limited mixing, lower.
20% of NYC can have the virus and the general level for herd immunity can still be what is postulated. NYC is incredibly dense compared to the rest of the USA and as such will naturally have more mixing and a higher threshold than say Topeka, Kansas