The paper is entirely anecdotal. It's a small, self-selected group who also selected when they would be tracked. I do believe that the group's worse performance was related to their laptop usage, though. So a third variable, perhaps a quality of the students, caused them to do the observed behavior (have a laptop, sign up, log into the process and then) and do worse in their courses.
Finally, doing slightly worse on two hour gen eds is also not the same thing as doing worse in general. Those classes are ones students are forced to take, often with professors who are forced to teach them. I know my gen eds were lower quality classes, with more people, less participation, worse teaching, and worse assignments.