No, I'm almost always serious.
To answer your question: A neck snap is a move taught in pretty much any martial art that has grappling tecniques. For someone as trained as Smith it shouldn't come as a surprise for him to apply it. Also, the neck is a very sensitive part of the body. A lot oess tougher than the torso or even the head. So if he's unable to hurt Zod in the body then it would make sense for him to target more vulnerable areas
And the kryptonians have disproportionate durability because out of all those punches and kicks and hits they delivered not one of them showed any injury (except the neck snap of course). Compare that to every other living being we know of that gets into fights, all of them have enough strength to injure another of their kind within similar size/weight range.
Heck, even a child can get your nose bloody if he hits it cleanly enough.