Why would you say that? Antifa (and the organizations it descends from) have existed and acted, sometimes violently, in many situations unrelated to the KKK or white supremacists.

For example, as we saw in Berkeley not long ago, they'll show up to violently protest Milo Yiannopolous by chasing down people in the street and using sticks to beat them unconscious.

(For the video, watch the first 2 minutes of: https://www.youtube.com/watch?v=oAqBxMZ-Zs0)

Of course they call Milo a Nazi. But calling Milo, the gay Greek half-Jew with a black boyfriend, a white supremacist just indicates that they're willing to stretch the definition to demonize anyone they don't like. If the KKK didn't exist, they'd stretch the definition as needed to recreate the KKK in their own minds.