I will disagree with that statement. What would your response be, pull your own knife just to keep the playing field level? "Officer, he pulled a knife on me--there it is--so I shot him tp protect my self from his deadly attack.Like CTR says, you can't respond to force with disproportionate force. If someone pulls a knife out and brandishes it, saying "Don't mess with me, I have a knife," you can't shoot them. If they are aiming to attack you, then you probably can.
A deadly weapon--and a knife IS a deadly weapon--justifies deadly force. It's not like some madder-than-hell woman slapped the snot out of you so you shot her. You pull a knife and I'll pull a gun.