It's like a square and a rectangle. All Nazis are fascist, but not all fascists are Nazis. It's just like how I'm an environmentalist, but I'm not a member of the Green party. It may seem pedantic, but I find it's important to be exact these days because fascists love weaselly word games. Kirk was not a member of the National Socialist German Workers Party, but he shared the majority of their general beliefs and behaviors and those are undeniably fascist.