Have you looked at the actual report and not the anti-vax site summation of the report?
>. The 21% lower incidence in schools that required mask use among students was not statistically significant compared with schools where mask use was optional.
I'm not sure how 21% lower is considered "not statistically significant", in trying to suppress the spread, ANYTHING > 0% is helpful. Full stop.
Other stand out qualifiers from same report
>This finding might be attributed to higher effectiveness of masks among adults, who are at higher risk for SARS-CoV-2 infection but might also result from differences in mask-wearing behavior among students in schools with optional requirements. Mask use requirements were limited in this sample;
>The findings in this report are subject to at least four limitations.
> * First, many COVID-19 cases were self-reported by staff members and parents or guardians, and prevention strategies reported by administrators or nurses might not reflect day-to-day activities or represent all school classrooms, and *did not include an assessment of compliance* (e.g., mask use).
>* Second, the study had limited power to detect lower incidence for potentially effective, but less frequently implemented strategies, such as air filtration and purification systems; only 16 schools reported implementing this ventilation improvement.
> * Third, the response rate was low (11.6%), and some participating schools had missing information about ventilation improvements. However, incidence per 500 students was similar between participating (3.08 cases) and nonparticipating (2.90 cases) schools, suggesting any systematic bias might be low.
>* Finally, the data from this cross-sectional study cannot be used to infer causal
relationships.
Basically was relying on self reporting. If a student contracted and was asymptomatic, not shown here, etc.