So 9% of seropositive and 10% of seronegative reported at least one symptom beyond 4 weeks. I.e. the background level is around 10% and the seronegative had "more" long covid than the ones who actually had covid in that group. Although this is pure randomness with such close measurements.
It is pretty weak though. There were only 109 seropositive kids in the follow-up according to the table, so you're talking about 2 more kids reporting a cough or headache as the entire basis of your concern.