Melting point of steel (roughly) = 1500 C
Kerosene (adiabatic) fire temp = 2200 C
'Office' fire temp (roughly) = 700 C
“jet fuel can’t melt steel beams” - Not sure where that comes from. It can. However if one applies energy equation you may find it only raises the temperature for the whole floor by about 250 C. Hence total possible temp = 700 + 250 = 950 C. Not enough to melt the steel. Melted steel observed in the footprint ...
It is true that both Young's modulus and yield strength would rapidly decrease with increased temp. However, kerosene would quickly burn out. Etc. etc.