Remarkably good footage, but it does not totally support your position.
Note that the majority of those bullet strikes presenting back splatter are into substances such as steel or glass or high-impact plastics. A great deal of heat is generated on impact, partially melting both bullet and target. Much of the back splatter is the result of the vaporization of either object. Note that the only identifiable metal objects in the back splatter are bits of the bulet jacket and a bit of the stil-solid lead. This spreads mostly parallel to the direction of travel of the bullet. That which rebounds in the opposite direction is largely dust or vapor or atomized liquid. There is some delay in the passage of the bullet through the target, and all of that liquid material has to go somewhere.
In this case, it can only go back up-range. Once the bullet has totally penetrated, most of the material folows it down-range through the hole. Not all of the bullet continues through because it is ablated in passing through the target, and, as a result, the hole is of a smaller diameter than the rear-most portion of the bullet. Thus you see the little medallions of bronze falling off the target.
For most practical purposes, the towers, being just bolted together where the planes hit, more resembled the paper targets than the steel ones.
Note, too, that there is no considerable back splatter from the bullet hitting either paper or ballistic gelatin.
Back splatter in the towers would, if you follow what I am saying here, require that the bolted joints of the perimeter columns and the aluminum cladding would offer sufficient resistance to either cause the formation of a small pool of vaporized metal or to cause the aircraft or parts of it to stop or bounce off.
Such appears not to have been the case. The kinetic energy of the impact of the aircraft was sufficient to shear the bolts on impact. Thus, there was no delay in the passage of our multi-ton bullet and no considerable vaporization of metal.
The Pentagon was quite another issue. That wall offered considerably more resistance and actually stopped the outward, lighter parts of the wings. It also overpressurized the cabin, thus causing parts of the fueselage to blow off just like the bronze jackets of the bullets. Most of the material, however, still had sufficient forward momentum to continue down-range.
I do not recall seeing any of the sequences in the referenced video that showed lead pellets striking light guage sheet metal. Sorry, but this fails to support your point.
Ignoring the dumb is harder than I imagined. Like driving past a car accident and not staring.