As you say, it might be true. Or they might have been reptilian aliens from the planet Zog who were beamed off the planes just before they crashed. Covers the known facts just as well.

But I can't see why there's any need to think up fancy explanations. Some people who believe strongly enough in a cause have always been willing to die for it, and normally they've been quite willing to kill for it as well. "Give me liberty or give me death". And having a partner and a family back home is a factor that always makes people less available to volunteer for any kind of suicide mission. (And that would apply regardless of the sex of the partner.)

Actually there was a report that, based on the presence or absence of goodbye notes and such, it could be that only the actual people who piloted the planes knew it was a suicide hijacking. Though of course that might have been some kind of cunning leak by the authorities designed to have some kind of propaganda effect.