There are multiple species of authoritarian tyrannies; fascism is just one of them. I think people get in trouble trying to generalize and apply fascism to places it doesn't fit. "Communist are fascists" might be one of the text book instances of that problem.

Mildly amusing thingie - a Bulgarian dissident's dissidenting involved writing a book about fascism (titled, for clarity, Fascism) which was really an oblique critique of the communist regime. It got past the censors and was published and then fairly quickly recalled. He later became Bulgaria's first post-communist president.