Unless Marxists have a different definition of fascism than the fascists have?
In a way, yes. Trotsky had his own definition of fascism, for example. He analyzed it as a historical movement beyond what individual fascists preferred to call themselves. In this way, Italian Fascism and Nazism were specific brands of the fascist movement Trotsky was talking about.
☭ “The ideas of the ruling class are in every epoch the ruling ideas, i.e. the class which is the ruling material force of society, is at the same time its ruling intellectual force.” - Karl Marx ☭