And the concrete in the towers was not reinforced but simply 100mm thick slabs poured for each floor.
The slabs have nothing to do with resisting the urge of the building to collapse under gravity. That is the job of the steel columns, concrete columns, concrete footings, and concrete shafts. Every skyscraper on the planet have massive amounts of concrete in them, even all steel structures. The footings are always concrete.
But the 100mm thick slabs most likely still had steel bars in them. And so do the buildings that burned in the examples I gave you. Most likely they had metal deck flooring with concrete pours. Sometimes the floors have prefabbed concrete floor planks. And of course they are supported by either steel beams or steel trusses.
No modern skyscraper consists exclusively of concrete columns, beams and flooring throughout or steel for that matter. Not even the Empire State Building. And certainly not the burning examples I gave. There is almost always a lot of steel and concrete combined.
Don't for a second think these buildings that burned and did not collapse were much different structurally from WTC 7.