IIRC, 14 of the 16 survivors where on that staircase. They were held up because they stopped to help an elderly secretary and thus in the right place at the right time to survive.
This is not that staircase. The staircase being discussed here was outdoors and went from the street to the elevated WTC outdoor plaza.
I used to walk up it every day on my way to the ferry, to cut through the WFC by the water.
On 911, thousands of people moved down this staircase to get away from the hell zone.