Pretty sure that most people know it was an American achievement and if the movie's chronicling Armstrong's life that'll be obvious as well, so not really seeing the damage done by omitting it. A movie about Iwo Jima isn't going to be harmed or somehow revisionist to leave out either of the flag raising as how significant is that to the battle ?
People saying the movie does show the flag on the moon's surface in many shots so, if true, it's even more of a non issue.