I actually read it as "They're no more unhappy without than with them" . Kinda occam's razor of if they're not unhappier without their phone, then there's no harm taking it from them.
I'm saying When we remove their phone, they do not have any sense of loss (unhappiness). If that's true then the simpler system of no phone is preferable to the complexity of having one.
If the phone was adding something (something unique/irreplaceable) then we'd expect to see some negative outcome once it's removed.