I have heard this sort of thing from zombie experts before. They tend to favor .22 for some reason because it was in a book. An melee weapons? Seriously? Over a machine gun? Give me a machine gun AND a melee weapon. No, the flat out best thing for anti zombies is going to be the .223 round. Small enough to carry lots and fast enough to punch through the skull at any angle.
A .22 generally lacks the force to exit and just ricochets around inside the skull for a while.
Maybe that’s why....:)