From the paper:
Eye tracking can provide insights into users’ behaviour while using the search results page, but eye tracking equipment is expensive and can only be used for studies where the user is physically present. The equipment also requires calibration, adding overhead to studies.The work is just exploratory -- concluding only that mouse tracking "definitely [shows] potential as a way to estimate which results ... the user has considered before deciding where to click" -- but does have interesting results on the relationship between mouse and eye movements. It also reports on common mouse moving patterns they saw searchers do when considering on which search result to click.
In contrast, the coordinates of mouse movements on a web page can be collected accurately and easily, in a way that is transparent to the user. This means that it can be used in studies involving a number of participants working simultaneously, or remotely by client-side implementations – greatly increasing the volume and variety of data available.