I think this is a situation where having loss of smell is a strong indicator of having COVID, but not having loss of smell doesn't imply testing negative.
Exactly. Losing your sense of smell is a strange and specific enough thing that, in this study, 80% of the people who volunteered because they noticed a loss of smell tested positive for COVID. But that doesn't mean 80% of people with COVID lose their sense of smell - other studies report "that the incidence rate of olfactory dysfunction in COVID-19 patients varies from 33.9–68%".
