It's not like they're sending architectural engineers to die in pedestrian human-bomb attacks. They just do it in the cases where the operative needs to understand/learn how to package, nitrate and detonate a crude explosive under stress, or learn to fly a plane.
Only through selective reporting do engineers appear over-represented.
In different times and places there have been many terrorist organizations who are supporting different kinds of causes. Engineers are significantly overrepresented in right wing religious ones, and are virtually nonexistent in various left wing ones.
Were it a simple case of selective reporting and economics then you would not see a correlation between the profession involved in the terrorist organization and the cause that organization is for. But there is a strong correlation, and therefore something deeper is happening than us noticing engineers because their skills and economic status make them useful to terrorists.
The article went only as far as suggesting that right-wing ideologies appealed more to the engineering mindset. And I'm hardly convinced that any given extremist ideology is more, or less, seeking to bring order and unambiguity to the world. That the summary went on to note that fundamentalist extremists were actively recruiting engineers didn't exactly help the argument that the over-representation is a natural result of mind-sets.
All things being equal, perhaps there are more engineers willing to kill for Allah than ELF. But the summary in that article is so unconvincing that I'm not particularly motivated to spend time with the paper.
When people ask why so many terrorists have engineering degrees, they really mean why do so many successful terrorists or high-profile terrorists have engineering degrees.