As I recall the antibodies are only reasonably detectable in a lot of people up to about 3 months after infection, after which the tests don't find them. On top of that we know quite a lot of people don't seem to end up with antibodies after the acute phase as well. Alas the article lacks the detail to assess whether they missed them due to being too late after infection or if there is a link between lingering symptoms and a lack of detectable antibodies. I wouldn't discount these people are ill its far more likely an error has been made in data collection or its a marker for the type of people with lingering issues. Because very real medically concerning results have been seen in long haulers (and ME/CFS) patients that explain a lot of their symptoms just not why they have POTS and cellular energy issues.