From the article, FWIW:
Studies suggest that while most people stop testing positive on antigen tests sometime during the first 10 days of their illnesses, a notable subset of people continue to test positive for longer, for reasons that scientists do not entirely understand.
In some cases, these people may still be shedding infectious virus, but in others, the tests may be picking up viral debris from a waning infection, experts say, making it difficult to know how to interpret the results....
...scientists disagreed on the best course of action for people who test positive for more than 10 days. While some say that the most prudent path is to continue to isolate, others argue that prolonged isolation is unnecessary for most otherwise healthy people.
Given the uncertainty, some experts have advised that test results at the end of an infection be viewed as just one potentially useful piece of information considered in concert with other factors, including a patient’s symptoms and immune status....
Sounds like a crap shoot to me!