Digital textbooks can offer so much more than print textbooks. They can have embedded interactive content, ranging from extremely zoomable 2D content to see something in detail to rotatable 3D models. They can have videos. Or just audio, say to listen to case studies or know how a bird sounds. They can have automatically gradable quizzes and exercises. There is so much more digital books can offer.
Once you bring in these advantages, I bet the pedagogical advantages will be enormous. I'd like to see that study, and I bet digital books will blow everything out of the water.
There are advantages apart from learning benefits. Errata can be a thing of a past with updatable textbooks. You can search for every single word easily and save time. There's no wear and tear. Students can get away with having textbooks on their phones, avoiding carrying textbooks weighing several pounds. Another huge benefit is accessibility. From large font to audio output, all accessibility features ever possible on a computer are possible with the textbook with minimal effort.
The publishing industry is only limiting itself because any innovation on its digital books will mean death of its money maker. It will mean much less production and distribution costs which will put in question its atrocious pricing.