Also, it might have to see with the level of blue light that is emitted by those devices. Light in backlit screens is somewhat filtered and might not contain as much blue.
In my experience backlit screens contain a lot of blue. I have been able remove the waking effect of my screen entirely using a heavy red filter provided by f.lux. But with the default settings, and even Apple's night shift implementation it's very blue. It is slightly less bad on newer devices that support "TrueTone".
"The randomized, crossover protocol design consisted of two conditions: (i) reading an LE-eBook (iPad; Apple Inc., Cupertino, CA) in otherwise very dim room light for ∼4 h before bedtime for five consecutive evenings, and (ii) reading printed books in the same very dim room light for ∼4 h before bedtime for five consecutive evenings"
In other words, they compared using a tablet to reading a book. Unsurprisingly, they found that using the tablet negatively affects sleep.
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Note that maximum brightness on an iPad is intended for reading in direct sunlight.