The point is a blow of such magnitude, in real life, would not knock back the target, it would instead, literally, open a hole in the body of the victim impaling him on the fist of the puncher, if the victim is not invulnerable . If he is, and the assailant is too, they both would fly in opposite directions destroying everything in their path since the kinetic energy would not have dissipated in any way.
It is the case of the irrestible force that encounters the inamovable object