Which was seriously remodeled by Large Falling Objects a few minutes after that conversation.
I'm with you. That wasn't shock, it was inertia and callousness. The guy was getting off his shift ("I'm walking"), and he was determined to wrap up and go home and leave the pillar of smoke for other people to handle.
On second thought, those conversations had to have occurred in the 15 minutes or so after WTC 1 was struck, and before the second airliner rained down debris on his building (one of the engines came to rest a couple of blocks to the north, on a sidewalk). For this guy to have made those comments to someone after the top of his building was peppered with plane, building, and body parts, he'd have had to have had the composure of one of Schwartzenegger's cyborgs.