There's no hard and fast cut-off for where pediatrics ends and adult medicine begins, but 22 years of age is one commonly used threshold. Many people in their early 20s (especially college students) are still seen by pediatricians and considered to be part of the pediatric patient population. When people in their late-teens to early-20s show up at hospitals they are often triaged through pediatric emergency rooms and admitted to pediatric wards. So that's where the 22-year old threshold in that article comes from, it's not "inexplicably define[d]."
But your point is still taken that when it comes to spreading the virus and the relative risks and benefits of in-person school, there is a HUGE difference between elementary / grade school and high school / college. Lumping all pediatric patients together as one doesn't capture this distinction.
See, e.g.,
In many places, the debates over reopening are fraught. But in a survey, experts broadly agreed that elementary schools didn’t need vaccines to open safely.
www.nytimes.com