You are way off on this one
You're suggesting two contradictory things.
1) the temps in the subbasement remained colder than the outside, explaining why all the thermal energy was transferred into the subbasement.
2) subbasment temps liqufied some surrounding steel because that’s where all the thermal energy transferred too.
You're suggesting all or most of the heat created from fires transferred down into the subbasement because it was colder than that of the external environment. But as the temps rose in the subbasement, far before they were sufficient to melt steel - 50-100-150 degrees - the subbasement became much hotter relative to the outside environment. Therefore, the remaining thermal energy would have transferred outward into the relativily colder external environment precluding formation of temps sufficent to liquify steel in the subbasement.
It's as simple as that.