On one hand, this argument seems intuitively plausible. However, the sample size is too small, and there is insufficient detail on how they controlled for other factors such as different teachers and environments. If I were part of the review board for a conference, I would decline to approve it.
Just for context, there is a mountain of research into the more general phenomenon of "desirable difficulties" in education -- practice conditions that make the task harder in a way that improves long-term learning. The increased difficulty is often mistakenly interpreted by learners as indicating that they are not learning as well when in fact the opposite is true. https://en.wikipedia.org/wiki/Desirable_difficulty
Spacing is one example of a desirable difficulty; the spacing effect has been known for over a century and it has been replicated so many times since then that it might as well be a law of physics. https://en.wikipedia.org/wiki/Spacing_effect
