If it's not too humid, you can start a regular campfire or bonfire in a pit for another day, sometimes two, just by turning the ash. They sometimes find burned roots still hot enough to easily ignite, long after forest fires have died out, too. With enough fine debris to keep warmth and air trapped in there, that's really not a huge surprise, given that there was melted steel to begin with.

The real question is what got it that way in the first place. The blast furnace gave us reliable steel production to begin with, and most high rises are practically blast furnaces waiting for a good burn. So, no, I'm not pulling a midwit, "jet fuel can't melt steel beams," thing - we melted steel with worse starting fuels for centuries. Rather, all that burning started from the jets was going on way up high in the building. If it collapsed right into itself, why did so much of that furnace interior stuff get so deep into the rubble? Unless, just maybe, it had it's way paved by a simultaneous demolition, that opened up the middle concrete and steel, or the fires were from such demolition.