It looks like it has much more to do with distractions and content type than the quality of the screens or fonts. From that, I assume that reading a good book on a digital screen with notifications off would show very little difference in comprehension. So, as usual with studies, the article title is a little misleading.
Screens are very very likely irrelevant, they did not control at all for content of the text. It's so incredibly logical that reading a full book gives better reading comprehension than reading tweets/reddit/tumblr/4chan to the point that this study is just one of the "duh" studies that may be necessary to say "yeah we already knew that but now it's confirmed"