If by "impact" you mean "the specific instant the tip of the nose cone touched the wall" then the plane did not disperse and disintegrate "upon impact." It was still whole and intact.
If by "impact" you mean the entire event between the tip of the nose cone touching the wall and the pieces all coming to a stop, which took place over a significant (in physics terms) period of time, then it's misleading to say "upon impact." "During impact" is more accurate, because it was during that process over time that the disintegration and dispersion took place.
Much of the dispersion and disintegration was in fact caused by the very same physical mechanism -- collisions and momentum transfer between parts of the plane and the parts in front of them -- that also drove a portion of the mass including the front landing gear deep into the building interior and through several relatively weak partition walls and the exterior C Ring wall.
This is evidenced by the fact -- which conspiracy theorists are fond of pointing out -- that relatively little of the plane's mass dispersed outside the building. The momentum of the fuselage and everything in it was all pointed in the same direction, which pointed through the round exterior hole, so everything in the way that contributed to dispersing the debris was also imparted momentum in that direction.
Just because the plane did not pass intact all the way through the building and out the other side, does not mean that it disintegrated into confetti the moment its nose touched the wall instead. Over time, it penetrated and damaged the building, and in the very same process over time, by the very same well-understood physical mechanisms, it was disintegrated and dispersed.
Respectfully,
Myriad
