It seems to me this paper is bringing to light the idea of spaced repetition for learning and this has become quite popular in the productivity/learning culture of today (e.g. Ali Abdaal).
> Alternatively, mathematics textbooks could easily adopt a format that engenders spacing.
I tutor middle school students in mathematics and this is definitely being implemented in their textbooks! At the end of each chapter, there a is normal chapter review practice test followed by a "Cumulative Practice" which reviews topics from the previous chapters in the book. These are especially beneficial to my students as, like the paper highlights, it promotes long-term memory of those topics.
> For example, although computer-based instruction typically provides extensive retrieval practice and rapid feedback, it offers a currently unexploited opportunity to schedule study sessions in ways that optimize long-term retention.
There is an immensely popular software called Anki which implements exactly this "spaced repetition"-type protocol.