Emphasis mine
I would guess because of different social distancing practices.
> Why would knowing who has antibodies hurt the economy when everyone is already secluded?
Knowing who has antibodies is going to be conflated with who has covid-19, because the general public is not going to get the nuance. And everyone is secluded right now but that will change.
However, I don't understand why you are asking me to prove the premise of the parent poster. I thought I was answering the question why he might have been downvoted, not whether what he was proposing was feasible. Maybe you could explain why you are asking me this. It seems you agree that a map targeting certain groups of people as likely of having covid-19 would be a bad outcome, isn't it? If so, you could probably understand why he might be downvoted.
And to answer your question, you see, there are these people called reporters, and they get a lot of information from very different sources. They filter out what is interesting and broadcast it to their audience. Perhaps you've heard of them.