That's not what the study says. They found that the seropositive group of 6 to 16 year olds was more likely to report a symptom beyond 12 weeks than the seronegative group. 4% of the seropositive students versus only 2% for the control group.
The study authors conclude that the prevalence is low, but they do not conclude that it's about zero.
For comparison, Polio is estimated to be asymptomatic or extremely mild in 90% or 95% of cases. Polio only moves to the central nervous system in about 0.5% of cases, and only about 2-5% of children with CNS infection die (15-30% of adults with CNS involvement). That's a relatively small percentage, but at scale it's a devastating number of people impacted. At epidemic scales, you can't simply waive away small percentages in the 2% range as inconsequential when it translates to millions of people impacted.
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.
That's not what the study says. They found that the seropositive group of 6 to 16 year olds was more likely to report a symptom beyond 12 weeks than the seronegative group. 4% of the seropositive students versus only 2% for the control group.
The study authors conclude that the prevalence is low, but they do not conclude that it's about zero.
For comparison, Polio is estimated to be asymptomatic or extremely mild in 90% or 95% of cases. Polio only moves to the central nervous system in about 0.5% of cases, and only about 2-5% of children with CNS infection die (15-30% of adults with CNS involvement). That's a relatively small percentage, but at scale it's a devastating number of people impacted. At epidemic scales, you can't simply waive away small percentages in the 2% range as inconsequential when it translates to millions of people impacted.