We knew full and well what the buildings in Tokyo were made of and what the effect would be. We'd seen the effects after another incendiary raid on Kobe on February 4 showed similar results, and a much smaller raid on Tokyo on February 25 leveled a square mile of the city. On March 9, 1945, we sent over 300 B-29s loaded with incendiary bombs SPECIFICALLY to burn the city. The March 9th raid still stands as the most deadly aerial bombing of all time. More than Hiroshima, more than Nagasaki, more than Dresden.
Interesting that they rarely mention those raids.