They have used an example which doesn't correlate with the tests in this country. It was precisely the example that they use which was the reaso why the UK waited for a test with greater specificity which means the likelihood of a false positive is far far less.Antibody tests used to determine if people have been infected in the past with Covid-19 might be wrong up to half the time, the US Centers for Disease Control and Prevention said in new guidance posted on its website
https://www.mercurynews.com/2020/05/26/cdc-coronavirus-antibody-tests-could-be-wrong-50-of-the-time/
@True_Blue69 read this properly