I assume that whoever coined the term was inspired by the (disputed) term clerico-fascism, referring to fascist movements that were not theocratic but were strongly supported by and influenced by the Catholic or Orthodox churches, eg the Utashe, Dolfuss, Franco, the Iron Guard. It's hard to think of Islamist regime with a similar structure- Iran is much more directly theocratic and not a one-party state. If the Egyptian Muslim Brotherhood were to do away with elections, that might be the sort of regime it would make sense to call Islamofascist. As a description of al-Qaeda, it's a "slur," however richly they deserve to be slurred, because it's just tacking on fascist as an insult without regard for accuracy.