Interesting read, especially as it relates to the product, I'm currently developing. My theories are twofold on this besides what the researchers mention.

For digital content, could this be because our brain deems digital content as lower "value". You consume both high value and low value digital content in the exact same way giving your brain a harder time to understand what is high value and low value, maybe the brain takes a conservative approach on digital content being low value unless proven otherwise. Whereas for physical books your brain attaches a higher value to this and thus you're more alert, maybe?

The second element is the difference in time between turning pages versus swiping on a tablet. Do these extra seconds give your brain more time to digest the information from the previous page(s)? This reminds me of a conversation, I recently had with a friend how TV shows 20 years ago were much more memorable. You watched just 1 episode a week from your favourite show with ads compared to binge watching a show with no ads nowadays. Our theory was that this gives your brain the time to process and digest the new information better.