Well, I guess they were wrong.
I can state categorically that you CAN ride on the hood of a Ford Mustang while holding a shotgun going 70mph.
Been there, done that.
The Mythbusters are right, but with the windows down, two strong guys can handle anything the driver can throw at them.
A friend and I used to play that game, and no one ever threw us off, not at any speed or maneuvering, and some guys would really get wild with their efforts, especially on the freeway and the grassy hills at the overpasses and switching to feeder streets.