I believe it was attached to the bumper. The bumper came off. For the chassis to break you would surely have to smash the hell out of it. I'd be questioning US regulation if you saw a chassis break like that.
Discussion
Nope, watch the whole Whistlin Diesel video. They DID smash the tow hitch with a large perpendicular force prior to the breaking, but it was in fact the cast aluminum frame that broke. While this was a torture test with conditions not encountered by regular users, the failure supposedly happened at least once in the field with someone whose trailer hit a pothole. Since cast aluminum tends to crack rather than bend, this has the potential happen even more when the trucks are subjected to smaller stresses over time as people tow.