Mags couldn't hold back Shaw because he'd just absorbed nearly the entirety of a nuclear subs energy core and he absorbed all the kinetic energy imparted by each impact. So every hit was not only useless against him, but only made him stronger. Kryptonians don't do that.
As for the knife, logic would dictate that kryptonian knives are able to cut kryptonian flesh. There's no point in making a weapon for use against your own race, only to have it be completely ineffectual.
And vs rules state the combatants know at least the basics about each other beforehand.