Is there a reason you don't think their numbers can be extrapolated to other places? Last week they were at 15% in NYC, and this jived with the numbers form other places, like the U.K., and I think also Germany.
It seems that as we get more data we're find more cases, and in increasing amounts, across the board. Which lowers the death rate and means we can reach herd immunity sooner than we thought.
I wrote that it should be mandatory IF it is needed.
Otherwise it's like saying that lockdown is needed but that it will only be opt-in: it does not make sense.
By the way, no, 1 in 5 people do not have antibodies.