True it could have happened with that case as well, however, the reason the screen cracked was most likely due to the concrete impacting the top or bottom edge, which are exposed with those sort of cases like the rock, nillkin etc. with the TPU cases these edges are covered. Also the diztronic TPU matte case is more of a rubbery case, not hard plastic like the rock therefore it has some form of shock absorption, which would also have helped for impacts like that.
You can't deny that the TPU diztronic case protects the device better overall and the chances of things happening like that are less likely than with rock etc. cases.
Only reason to go for the rock, nillkin cases etc. is to keep the sleek, minimal look for the front of the phone.
Also do you know how these people got their screens cracked when using a case covering all edges? For all we know, they could have dropped the phone and been extremely unlucky and had the screen hit the corner of a desk or the corner of something else in which case all cases are useless apart from flip cases and thick/hard pouches.
Besides the iphone isn't exactly good for durability anyway even with a case on
Pretty sure it would stop the camera lens touching the desk surface etc. as the GS 3 camera lens is slightly sunk below the rest of the mobile anyway (IIRC), where as with my S, the camera lens sticks out by about 1mm and is now below the surface of the diztronic case (you can see my review of the diztronic case
here