I got an email from a friend who spent a summer working for TX Parks and Wildlife doing a herp survey of that park. While there he learned of several reports of this species in the area and there are other credible sightings as well. (He also happened to be there that day and identified the snake in the photo for the birder). It may be the northernmost population of these beasts.
Fortunately, most of the habitat is private (inaccessible) and very rugged so it remains out of the hands of most destructive farming practices. And there are areas of protected land as well.