Sergeant Simons said Armistead died behind the lines, which is accurate. He said nothing about when, which he would not be expected to know while the battle was still in progress, as his place was with his battery. If his gun was toward the left of the battery, he might have been able to glance over the corner of the wall and see dead or dying Confederates to his left and front. Simons seems to indicate that he did not personally hear Armistead, but the latter's words were conveyed to him by the men he dispatched to comply with Lt. Weir's order.
I can envision the fellow standing at the wall in the top photograph of the below post, by Gettysburg Greg, as being very close to where Sergeant Simons stood at around 3:30 p.m. on July 3:
https://civilwartalk.com/threads/comparing-the-cyclorama-to-1882-photograph.134316/#post-1535247