Well I don't think that is the case. Parents and teachers are observing students not paying attention.
I would think if an educator can teach a full lesson, while also ensuring that students retain the information, when the student is watching YouTube, endlessly scrolling reddit or lemmy, or on Instagram this wouldn't be an issue.
The problem is that students aren't retaining the knowledge being provided to them.
Here's the thing, in general. Phone or no phone, the problem your mentioned is down to the teacher. I've had good teachers that are able to make a lesson entertaining and guess what. That actually works, people aren't bored to death and shit gets done.
To contrast, boring lessons, even without factoring in phones, nobody gives a shit for good reason. We just focus on something else, wetger that's a phone or something else isn't a factor. Its rather the lesson itself
To address the retaining info part. If somebody talked to you for about half an hour on some random thing you don't care about. How much do you think you would remember or focus on. Spoiler alert, probably not much