Irrespective of the effects of inbreeding on any population, I respectfully submit that it would be a mistake to attribute fervent Islamic radicalism to a lack of education or intelligence. There is something else at play here that is much more unsettling.
I think that another sorry aspect of Islamism is that the "something else at play here that is much more unsettling" are those who take advantage of the victims of inbreeding.
Islamism's powerful faithful, mullahs and imams have their "72 virgins", their wealth and power here on Earth. They don't need the "heaven" urged upon the victims of inbreeding and the just plain poor and ignorant.
I do see a difference between Islamists (political Islam, sharia law advocates) and Muslims. I worked a few years with Muslims from Turkey including almost a year in Ankara.. years before the current Islamist AK Party government.