That's not what the paper linked to concludes. The study compared 5-year-olds who didn't wear masks to 6-year-olds who did. The conclusion was that age-dependent susceptibility statistically overwhelmed the impact of masks, so a conclusion about masks couldn't be drawn.
From the paper:
"The immunological innate host response in younger children that wanes as they get older,
alongside classroom dynamics, could explain the age-dependency gradient"
No, that is very much what the study concludes. Quoting directly from the abstract:
"FCM [facial covering or mask] mandates in schools were not associated with lower SARS-CoV-2 incidence or transmission, suggesting that this intervention was not effective."
In other words, if masks were effective, you'd expect to see exactly the opposite, namely that older (masked) kids get sick more often than younger (unmasked) kids. But per the data, the masks had no visible impact at all, and age was the actual primary determining factor.
Of course, if you've ever seen young kids wearing masks (poorly fitted, not covering nose, hanging around their chin, etc), this is not in the least surprising.
From the paper:
"The immunological innate host response in younger children that wanes as they get older, alongside classroom dynamics, could explain the age-dependency gradient"