So, I'm not getting why selection bias would matter here. If they are counting the total number of participants who were positive for the antibodies, and this number was out of the total number of residents in the county, big deal.

Why would they ask all the healthy people to come get tested? Just assume that all the untested people are negative for the antibodies.

Otherwise you would get a higher infection rate count, which would of course result in a lower mortality rate for the disease.