The buildings were supported by a massive steela nd concrete core which was essentially undamaged. Think of an umbrella. If you look closely at some of the film you can see the core standing through the collapse.
The building also survived long enough for nearly everyone who chose to evacuate immediately to escape -- except for those above the fire.
I think in twenty years, engineers will look back on this as a successful design, although with perfect foresight, it could have been better.