*Very* astute.
I had come to the same conclusion myself.
People are assuming that the terrorists "knew exactly" how to bring down the buildings. They're forgetting these scenarios:
1. The terrorists naively and wrongly assumed that the plane impacts alone would topple the towers, but they didn't. But they got lucky and the flames finished the job, much to their surprise.
2. The terrorists figured the best they could do was to cause twin "towering inferno" out of control fires (which is why they made sure they used fully fueled planes, and hit high enough that water from the ground couldn't be sprayed on the fire), which would be more than enough to cause enormous death tolls and major financial/business disruption. They simply succeeded better than they dreamed when the fires caused the buildings to fail structurally.
Other factors which point to less than masterful planning was the relatively small amount of damage to the Pentagon (and into the *least* occupied wing of the building), and the botched fourth hijacking.
I don't see this as necessarily being the work of criminal masterminds. Balls and sheer luck could have been all that was needed.
Finally, it wouldn't surprise me if all the structural advice necessary to do even a planned demolition was contained in the pages of Time and Newsweek in the weeks after the original WTC bombings -- remember how all the news magazines fell all overthemselves interviewing building engineers and posting breathless stories about how, "it could have been worse, things could have been really catostrophic had the terrorists instead done something more like..."