Their argument that digital textbook users retain less info is tenuous at best. I used a digital book for Calculus I & II and thought it was easy to use, convenient to carry around, and helped me learn more.
On the contrary, I find physical textbooks to be largely superior to digital mediums. You aren't limited by DRM imposed by the various educational companies. (I loved only being able to download and print 5 sequential pages at a time when you're looking at printing a page of questions for each chapter) I find it more engaging with using a real textbook. You can feel the pages, write in the margins, not be distracted…
Also underrated is how easy it is find find something in paper book, based basically on how thick the two sides feel in your hand.