I subscribe to the......... how the hell can you design a building to withstand the impact of a fully laden and fuelled 767 without testing.
The theory obviously didn't work.
It wasn't, it was designed to withstand an impact of a smaller airliner* at a lower speed, as it was expected that it would be preparing for landing, and lost. The design remit was that it could withstand such an impact, but the designers admitted to being uncomfortable about it withstanding such an impact even then.
The 767 was not only bigger, but flying at a higher speed, and fully fuelled.
For those that doubt the planes brought them down:
There are pictures that show structural deformation prior to collapse, which means the heat from the fires was intensifying to such a degree that structural elements were been weakened bit by bit as the heat intensified.
Aluminium was found to be melting and dripping onto the sidewalk, just to give an idea of how hot it was. In addition, the impact removed much of the fire retardant in that area, and the designers hadn't actually accounted for this, as the impact calculations simply looked at the impact and not the fire. The retardant was to handle a fire from a more conventional source, not a plane impact.
* Although strictly speaking, the designers looked at what they designed and worked out what the impact would be for a 707 after they had actually designed the towers not before it was designed. There may have been some modifications made after this, but it is unclear as to what they were.
The documentary was spellbinding.
Such bravery on that day by so many.