All scientific studies following the scientific method of looking for particular results to either uphold the hypothesis or reject it.
I, based on my own experience agree with the results. It's not saying that one is better than the other nor is it saying that it's black and white. I think there is a very broad spectrum of literature and it's not always easy to determine.
I think the primary thing they did in 'choosing' what literary fiction was vs non-literary was to choose character-based vs plot based.
I think there is a lot of overlap, for example how would you classify E. L. Doctorow's Billy Bathgate? Or some of John Irvings work The World According to Garp or ...
Probably don't learn much Science from Literary Fiction though. SF has got it all over literary fiction there.