Exactly. The paper itself concludes somewhat softer that "children may be a potential source of contagion in the SARS-CoV-2 pandemic in spite of milder disease

Exactly. The paper itself concludes somewhat softer that "children may be a potential source of contagion in the SARS-CoV-2 pandemic in spite of milder disease or lack of symptoms" The patient group consisted of "children ... presenting to urgent care clinics or being hospitalized for confirmed/suspected SARS-CoV-2 infection or multisystem inflammatory syndrome in children", so they are pre-selected , making the role of the full amount of children in the population in spreading harder to know. As you note, that doesnt rule it out either.