Obviously this has been quite a heavy impact to break off a huge chunk of magnesium, this was not "snap off" in any way or form!
This was a drop/impact with substantial energy, looks like a big heavy lens.
Magnesium is very stiff and quite strong, an extra 'tiny' mount screw would not have made much difference with such a high energy impact.
None of the Nikon DSLR has a mount break away design.
Some of the cameras are even space plastic for securing the mount,,, We had a long thread some years ago
Some lenses has a break away ring near the lens mount, 24-70mm AFS f/2.8 for instance, that makes a clean cut, however you can often see the front group, zoom, suffer under impact - Keep your lens hood on at all times to minimize the risk of front group/zoom damage.