It happens all the time. Causes lead to different outcomes in different environments. In biology for example poor diets lead to different diseases in different parts of the world.
Extremists are more likely to be poor. Some instigators may be well-off people doing it for whatever purposes, but your rank and file, your average suicide bomber is more likely to be idle, unemployed with nothing better to do
He is not positing a general theory of income inequality and extremism merely offering a hypothesis for recent extremism in the middle east. Saying "I think X is the cause of Y in country Z" is not the same thing as saying X is always the cause of Y in all countries all the time.