Over at Greg's place today I wrote about Marc Thiessen erroneously referring to Al-Shabaab as "al-Qaeda in East Africa" in an attempt to defend the pointlessly broad premise of Rep. Peter King's hearings on domestic radicalization. Not only is Thiessen wrong about Al-Shabaab merging with al-Qaeda (they're still wannabes, and a merger would be a significant development) but according to Christopher Anzalone, a Ph.D candidate at McGill who studies transnational jihadi movements (and blogs at al-Wasat), Thiessen also overstates the role of American Omar Hammami, whom he referred to as the "military commander" of the group.
In Shabab media, Omar Hammami has been referred to generically as a "leader" or a "shaykh," which in this usage also just means "leader." There is no strong evidence that I have seen that he occupies a position within the movement's hierarchy that is anywhere near as grandiose as "military commander." He has been shown observing training of Shabab "special operations" forces, though his exact role vis-a-vis them was unclear. He may be a military field commander, but he is one of many.
I suppose if you're Thiessen, and every Muslim militant group in the world is part of what Matt Duss calls the "Islamofascist Voltron" of the conservative imagination, these sorts of details don't really matter.