Looking over their data, it seems that they coded incomplete text as an error. So, for example, participants 34 and 37 completed none of the table exercise and were coded as having produced 513 errors (both were LaTeX users). This accounts for the vast majority of the variation in the observed errors. I think they've convincingly shown that Word is more productive than LaTeX for some of the study tasks, but not that the resulting output is more correct.

That being said, I don't doubt that LaTeX is harder to use and more error prone than Word. Since I find myself frequently writing mathematics-heavy text, I personally prefer LaTeX ...