No one said that.You don't need to be a ninja to defend yourself with a knife.
People use knives with some regularity to defend against dogs, mountain lions, etc., all without any type of specialized training.
There's a difference between an animal and a human. Huge difference.
I'd wager that the majority of folks who you a firearm, conceal carry or otherwise in a lawful self defense shooting haven't attended even one class at Gunsite or any other facility of that type yet they manage to come out on top.
Not saying training in either firearm or knife wouldn't have its benefits but it's certainly not mandatory.
A knife is an intimate weapon. The combatants will be close enough to smell each other. They'll be fighting for their lives and control, retention, of that knife. A firearm, gives time and space. That is given up with hand to hand and/or a knife. Is that a situation that anybody wants to be with no skills?
If the person is older and out of shape with no skills and they try to use a knife against a younger, faster, stronger, more determined opponent, an opponent who has basic skills, and (with all the MMA classes, there are a lot of people - good and bad- have basic skills) and have a knife with no idea how to use it, they are done. A no skills person is a huge danger to themselves.
If anybody thinks they don't need skills to carry a defensive/fighting knife, I say go to ANY martial arts class and spar with the beginners and see how that goes.