I think their results are explainable by the fact that they had everybody reading fiction.

Think about how nonfiction and reference (textbooks, manuals) text is often divided into two or more columns per page.
Ever notice how fiction is NEVER printed like that? Fiction is pretty much invariably printed in one wide-ass column going all the way from margin to margin.

My personal experience bears this out. I have a couple of fiction texts printed in 2 half-width columns (these are bilingual short-story books, with the same story in 2 languages printed in parallel columns), and, tbh, the narrow columns degrade the storytelling aspect. It reads more like FACT FACT FACT FACT FACT FACT, like a newspaper article.

Since smartphones are narrow, they force the same narrow-column format, which will really fuck with your ability to read fiction. 🤷🏽‍♀️ Is my hypothesis.

If they found that the same problems don't happen on tablets, and don't happen with nonfiction on your phone, that would be strong evidence for the lipsypothesis here

You make an excellent point. I have had pretty severe adhd all my life and struggle to pick up physical books. I end up looking at my phone every few seconds anyway so I got into ebooks. This has been great, because many have a feature that lets you format the text like an article, so it breaks it down in a way that visually is a lot easier for me to follow without getting distracted.