They’re reporting antibody statistics for those who had symptoms and got tested, and are assuming that is reflective of the broader population. Sounds like a mistake to me.
"On the other hand, among those who did have the illness (based on symptoms), it would appear that overall greater than 70% of them developed antibodies"
Unless im reading it wrong, many did not get tested for Covid19 and only had Symptoms.
I agree that it isn't reflective of the broader population, but one could argue that those same numbers would be reflected among the asymptomatic.