That's some pretty serious damage. I'll grant you that the damage to the lobby he observed was long after his stairwell collapse, but any event large enough to collapse the stairwell beneath him would have penetrated into the lobby with which it abutted. Don't forget, he described a "HUGE explosion". Additionally, the camera crew would have noticed any significant damage from a huge explosion. They were within spitting distance of the stairwell.
Ps: There was no basement in that area. The ConEd substation occupied the first 3 floors, and it was undamaged.
Undo what Wilson did