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.
What about even more phones? Keep the kids hooked up to a constant drip of tiktok 24/7
On the flip side, when measurements don't reveal something that is completely obvious, especially in social sciences, it is very possible the measurement is measuring the wrong thing.
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.