A related possibility is that most physical books are professionally typeset with line lengths optimized for comfortable reading. Here's a wikipedia article with examples.

The bilingual storybook is a great example of what happens when the line length is dictated by other factors.

I would also be interested to see if tablet/e-reader users fared better, assuming their device's text/column/spacing settings matched the paper version read by participants.

+1 to all this.

also i wonder whether anybody had the good sense to flip the damn phone sideways 😂