The many witnesses who ran up the grassy knoll looking to see where the shots came from are what is important.
They knew the bullets were buzzing over their heads.
They were greeted by fake Secret Service agents flashing ID badges telling them to move along....nothing to see here.
You dismiss Aaron Zapruder, who had a view of the grassy knoll. And he was former Army. He would know what a rifle shot, going off at the level of his feet, would sound like. And they never show it but the end of the film shows nothing behind the fence. Nothing.