"Engineers are more likely to be terrorists" is different from "terrorists are more likely to be engineers".

You can imagine that engineering is a useful skill for terrorism and thus terrorist organizations might spend extra effort trying to recruit engineers. They may also have a higher survival rate working on behind the scenes tasks rather than firefights and suicide missions, which could cause a survivorship bias in data collection.

(It's also interesting how many foreign leaders and dictators have engineering or science degrees, and/or went to US universities prior to becoming leading figures in their home countries.)