However he's making the fundamental error that the upwards force is equal to the downward force. Basically saying that upon impacting a floor, the upper block will stop entirely. That is total nonsense.

You have to look at the basic mechanisms of failure. There's basically horizontal structures (the floors) and vertical structures (the columns). Now the columns can carry a lot of weight, so long as they are both vertical, and braced at regular intervals. The floors cannot carry much weight. So if you drop a large mass on the floor, it will fail. This both removes the bracing, and pushes and pull the vertical columns so they will fail. The outside columns will fall away from the building as their lose floors, the interior columns will fail similarly inside the building, but more needs to be stripped away before they fail.

But these things don't happen at the same time for the same floor. Vertical structures would likely collapse only after several floors worth of horizontal structures have been stripped away. And they would require relatively small amounts of energy to collapse - in fact the exterior columns would have buckled and collapsed simply under their own weight, with no additional force needed.

So it's a bit more complex than his simple equations. You don't have a solid block falling onto another solid block. The initial floor-floor impact would compact the bottom floor of the top block as much as the top floor of the bottom block. But after that you've essentially got a falling mass of compacted floors that grows rapidly, getting bigger and bigger, doing more damage to the lower floor than the upper floors. Each floor detached adds to the falling block. The retarding energy acting on the top of the top block is approximately the energy required to break the horizontal members away from the vertical members, plus a small amount required to push the vertical members.

This is an old argument. The truther versions of it range from

simple incredulity to calculations to rely on incorrect assumptions (like in your linked video), to calculations that simply plug in excessively large amounts of energy required to "destroy" each floor, rather than simply separate it, loading to collapse with buckling.