I'm not sure I understand this, are you questioning why a system would teach an unarmed person to defend against an armed attack such as a knife?
Again, I'm not sure I understand this. If you train an art that spars you are training to defend an attack from an experienced fighter. Any sportive system like judo for example trains you how to defend against a skilled fighter. If you can handle a skilled fighter you can handle an unskilled fighter but the reverse is not true.
No, what I'm questioning is why he's saying he would defend someone who is carrying a weapon which is what he said. Personally I'd defend myself against someone with a weapon. the only people who defend armed assailants are the lawyers they employ after they've been caught by the police, you don't defend people who go around armed with intent to harm.