The antibody tests seemed to be consistently underestimating other measures of Covid-19 immunity, even conservative ones, so they're not considered as useful as we thought in April or May.

It's possible the tests aren't sensitive enough, or immunity is largely based on T-cells (more expensive to test for) rather than antibodies [0], or antibodies for other coronaviruses confer some level of immunity, or something else we still don't understand.

[0] https://www.eurekalert.org/pub_releases/2020-08/cp-mcc081720...