You do have to remember that the engine that render the game is not final, and so also for the vehicles. I know of at least one that was shown in the trailer that still had stuff to be done on it. About the "demo glitch", that is an interesting one.
I don't think it's much of a glitch, and there is actually a slight chance they released that screenshot to so many magazines and online sites on purpose. I mean, they obviously look through the screens before sending them out, and you spot that clearly within 2 seconds.
The guy on the left is likely Niko, and they could've gotten out of that car, climbed on top of the bridge, activated the free-aim camera and took a shot. So yeah, could be that they wanted fans to speculate you can even climb that good... Niko perhaps a former gymnast ?