What seems blatantly obvious does not always represent the underlying reality. This can also be taken as a chance to discover better solutions to a given problem than banning phones.
In this case the prevailing theory is that the phones were already banned in classes, so the schoolwide ban didn't do anything to screentime. Which, duh.
I’m a strong supporter of following the science. But this is a hard thing to measure. And the effects may be fairly diffuse or accumulate over time.
In order to quantify whether something is too much, you need to quantify what an optimal or better baseline would be.