Well, they have dominated Marxist politics for quite a while, and I am not sure you can say they are not leftists because they are authoritarian, they have a lot more in common ideologically with other Marxists/socialists than the right, just because they prefer certain methods does not mean they are not leftists, and they do have traits that have always been associated with leftism, like opposition to tradition, monarchy, even capitalism, desire for revolution, etc.

If Marxists are not leftist, I do not know who is.