I haven't really read it in detail, but in the abstract, see the sentence:
In order to rule out the role of personality, we first identified Openness as the most consistent correlate. This trait was then statistically controlled for, along with two other important individual differences:the tendency to be drawn into stories and gender. Even after accounting the tendency to be drawn into stories and gender. Even after accounting.
Which means that fiction was still predictive after accounting for various self-reported personality traits. So they did try to differentiate the two.
For detail, the corresponding section: "Association between print-exposure and empathy: Ruling out the role of individual differences"
This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous thread is at close to 500 comments.