The short answer to this is that Japan did not have anything like "impeccable costal defenses" by August of 1945 -- raids of American B-29s, sometimes numbering in the hundreds of planes, had been bombing Japanese targets from mid-1944 onwards. The strategic bombing raids increasingly shifted their attention to urban centers by the spring of 1945, and the strategic bombing was augmented by raids from carrier airplanes as well.

These bombing raids were often preceded with small flights of B-29s serving as weather spotting planes, meaning that Japanese military commanders were quite used to seeing single or small numbers of bombers over cities and would tend to husband their resources to combat larger raids.

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