I agree that this study didn't separate the two very well, but it's a difficult task to be fair.
They found that the kids' bed times were far earlier without phones, but was that a short term effect? Was it an effect of being observed and measured? If the parents valued their kids' sleep, why was the average bed time of 12 year old kids after 11pm pre-ban? You could blame that lack of sleep on phones if it made you feel better I suppose, but it's clearly not the whole story.
They found that the kids' bed times were far earlier without phones, but was that a short term effect? Was it an effect of being observed and measured? If the parents valued their kids' sleep, why was the average bed time of 12 year old kids after 11pm pre-ban? You could blame that lack of sleep on phones if it made you feel better I suppose, but it's clearly not the whole story.