There is a video at the bottom of this page that offers a ZOOM discussion with a number of historians.The guy who made the map based it on charts and lists that were made of the bodies and burials in the days and weeks after each battle.
His Gettysburg map identifies about 10 specific men by name. The one from Antietam identifies the graves of about 50 men by name. The historians make some connections to the Alexander Gardner photos from Antietam that were on display at the famous NYC exhibit in October 1862.
The men in nearly all of the graves on the Gettysburg map were moved to the National Cemetery or elsewhere.
That is also true of many, but not all, of the men on the Antietam map.