When you're dealing with large populations (here, the study include 230,065 students--a very large number), even small shifts due to some treatment can be significant. It is very hard to generate top-down policy interventions that shift the mean of a population in significant ways: if this treatment effect (banning phones) is real, 1.1 points represents a very big policy win that can easily be applied elsewhere. The devil is in the details, however: they exclude some recent data based on the pandemic, but baseline off of 2022-2023, which was still in the throes of the pandemic. The data they show looks to have around a 0.5-1 sigma variation in percentile from 2022-2024, so the shift from the baseline of around 1 to 4 definitely looks significant, but it will be interesting to see if sticks over time.