There are some issues with extrapolating that data to NYC.
"An analysis of the blood of some 3,300 people living in Santa Clara county in early April found that one in every 66 people had been infected with SARS-CoV-2. On the basis of that finding, the researchers estimate that between 48,000 and 82,000 of the county’s roughly 2 million inhabitants were infected with the virus at that time — numbers that contrast sharply with the official case count of some 1,000 people reported in early April, according to the analysis posted today on medRxiv. The work has not yet been peer reviewed."
This is only a sample so without knowing the methodology it's hard to know how representative it is of the whole county. If we assume that it is to a high degree, what it tells us is that 2,4%-4,1% of Santa Clare had been infected with COVID-19 at that time. That's the first thing.
Second thing, and this is where it gets even trickier, is using this relative comparison between confirmed cases and actual infected from Santa Clare and transferring it to the rest of the country. As the article says, based on the antibodies Santa Clare has an 48-82x amount of actual infected compared to diagnosed cases. But there's something else we have to consider.
https://paloaltoonline.com/news/202...ents-have-likely-been-infected-by-coronavirus
NY have tested about 600.000 people at this point, which is roughly 3% of their population. Of those, about 245.000 have tested positive, or 40%. California have tested about 280.000 people at this point, which is roughly 0,7% of their population. Of those about 31.000 have tested positive, so about 11%.
https://www.worldometers.info/coronavirus/country/us/
So we know that NYC have tested about 4,3x more of their relative population size compared to California, and almost 4x more have tested positive out of that pool. What this means is that NYC data is way more accurate than Santa Clare in regards to the actual number of infected, which makes it almost impossible to extrapolate the underreported data in Santa Clare onto NYC.
With that said, there is no doubt that NYC has A LOT of their population infected. Probably more than any other place in the world outside of Lombardi. Doubt it's as high as 70% though, but hopefully we'll find out soon with more testing. Putting aside that the antibody tests seems a tad unreliable atm.