The mass is the mass, the floors are not going anywhere. So you have a falling mass, that is continually getting heavier.
Things weigh the same regardless of if they are moving or not.
But only a static weight, not a moving one.
By mass they mean the block of the building. To get the weight you do mass x acceleration. As you can clearly see the bottom part of the building has more mass that the top part and MR chandler explains in the video that the force of interaction is a third of the weight that the top of the building would be if it was just sitting there. With the collapse theory It doesn't matter weather the building was designed to withstand a static or falling mass what matters is the weight of the mass. Because that is what the official pancake theory is claiming. That you know it pancaked because of the weight of it.