NYC had a 20% antibody rate in a sample of people who were out shopping and consented to giving their blood for the study. That is enough potential sources of bias that assuming the general population had a 20% antibody rate strains credibility.
People out and about shopping or working are also more likely to be the ones spreading the virus. So even though the 20% rate may be an overestimate, if the people more likely to be 'superspreaders' are now immune, that may reduce transmission.
NYC had a 20% antibody rate in a sample of people who were out shopping and consented to giving their blood for the study. That is enough potential sources of bias that assuming the general population had a 20% antibody rate strains credibility.