I've only skip read bits of it, but it seems they're discounting the conclusion that everyone jumps to: engineers skills are useful. Instead they're putting it down to the combination of two factors:
Firstly, the overrepresentation of engineers occurs in North Africa and the Middle East, but not in the West, so they proposing that educated engineers become frustrated and disillusioned in economies that have fewer opportunities for their skills.
Secondly, they note that engineers (in America at least) are significantly more prone to extreme right wing views.
The second point strikes me as question-begging. "Why are engineers terrorists" and "why are engineers right wing extremists" seem to be two variations of the same unanswered question, but I haven't read the paper in detail to see if the authors offer an answer.