hiroshima burned down in aftermath of bombing, the damage was nothing exceptional compared to firebombings of other japanese cities such as tokyo
and concrete/masonry buildings further than 1km from ground zero were practically undamaged, people inside also shielded from radiation
you can also read this report on bombing of hiroshima and nagasaki to convince yourself of those claims:
https://bernd.group/up/6f9c5f0100d6c9c874da0565c3e8afe0.pdf
in case you haven't noticed americans also traditionally build houses with cardboard
also this