Elsewhere I would agree with you but this study was using:

"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.