There is a psychological coping mechanism of conversion - effectively, it is easier for the mind of some folks to convert fear of an undefined (or under defined) stressor into aggression. It’s not so different than the variant reaction some people display when insulted, or punched in the mouth - some react with sadness, some with fear, some with blind aggression. The mind’s perception of a potential stressor, whether it’s an insult or an assault, for some folks, will parlay processing the primary emotional response and instead convert to something their mind deems as more controlled - anger and aggression. Figuratively, it’s the minds’ way of saying, “sometimes the best defense is a good offense.”
You see the same conversion response in people who “psych themselves up” before something scary. I’ve seen guys slap themselves in the face and scream obscenities before parachuting. Heck, this conversion coping mechanism is commonly relatable enough, it’s used in a scene in the kids’ movie Frozen - “I was BORN ready...” - before the protagonist jumps off of a cliff. It’s easier for the conscious mind to be comfortable with anger than with fear.
Doesn’t make it right, but it does make it common. So when your buddy seems a little aggressive whenever he talks about his preparedness for a self-defense scenario, it’s most likely a conversion of his inherent, underlying fear of being unprepared for a nearly “unpreparable” event.