This is all that is left? What is "this"?

If "this" refers to "the dust (mixed with unburned paper)", why does "the dust" only belong to the top half of the Twin Towers rather than the whole?

Context:

9/11/01 -- A Typical Street in Downtown Manhattan

Mostly unburned paper mixes with the dust of the top half of the Twin Towers. This is all that is left of the concrete floors, gypsum wall board, steel decking, office furniture, office machinery, filing cabinets. The dust averages 4" deep. Most of the 6" curb appears filled in.

More:

http://www.american-buddha.com/911.blueprintfortruthae3.htm