> I think you misunderstand their idea. A child with worse coping can push their parents harder for a phone, and a child with better coping skills can hold out without a phone longer. This is the theory they're suggesting, I believe.

I think you're reading that into their comment when it's not there. They didn't mention nagging at all, and even if the kid nags, the parent still has all the agency.

If there's some correlation-not-causation criticism here, it's probably some along the lines of parents with the fortitude to say no to their kids will raise more mentally-healthy adults, and successful phone restrictions are correlated with that.