I seriously doubt it. The area on the top-plate is totally tweaked and penetrated. You're probably not going to do that with a slingshot other than one of those crazy ones. While I don't disagree that the bullet probably wouldn't have killed him if it had slowed down by that point - but the point is his camera took a bullet while in the process of documenting a war. No reason to minimize that or get pedantic over it.
Nor is there any good reason to maximise and dramatise it to the point of unreality, hence my post.