What's bollocks is your assertion that the upper section "should have" decelerated. As it was FALLING, it experiences a constant acceleration straight down of g (-9.8 ms^-2). However, observed acceleration was only 2/3 of that, which means that there WAS a resisting force pointing up, that added a deceleration of about 3.3 ms^-2. And guess what: That computes to the expected average acceleration one would expect from the average strength of buckling columns, as they meet dynamic impact of mass m.
It also computes nicely when you consider available energy from gravity versus needed energy to deform structure for total destruction. The math has been done and published in real science magazines and shown to match observations.
You are thus demonstrably WRONG.