There's no indication in the article whether it was conducted with e-paper Kindles or the Kindle Fire. It's already well known that reading on computer screens decreases retention, so it seems like that might factor into a test with tablets too.
Why do you think it suggests that? (I'm not super familiar with the feature sets of Kindle models, but none of the above suggests LCD or backlighting.)
I'm mostly curious whether these results would be the same with an e-ink screen as with an LCD. I believe the paperwhite is front-lit e-ink, BTW.