The mechanisms of oppression and dictatorship are the same. But that doesn't mean Fascism and Communism are similar ideologies.

You can find similarities in how oppression works if you study Maximilien Robespierre's reign or terror. Idi Amins reign in Uganda. Gadaffi, Saddam etc.

But all these people had quite different ideologies. Not every communist is a wannabe autocrat.

One gets a false sense of similarity by comparing communist dictators with Fascist dictators, because a lot of what you see which looks like similar ideology is simply the similarties in how oppression works, regardless of ideology.