One more thing. I know these men were there. All accounts of Iwo Jima relates that the Japanese allowed the marines to land on the beach before they opened up on them.
Maybe they were not in the initial landing.
They waited for the beaches to be filled with men, that way, the maximum effect of the shrapnel would hit more men because they were all closer together relatively, not spread out where one shell might hit only 2 or 3 men.