"If the KKK claimed to a Japanese rights group would you believe them?"
Irrelevent. They are what they are.
"Antifa is fascist."
Antifa is anti Fascist, as in opposed to, a reaction against...
"Fascism (/?fæ??z?m/) is a form of radical authoritarian ultranationalism, characterized by dictatorial power, forcible suppression of opposition and strong regimentation of society and of the economy, which came to prominence in early 20th-century Europe"
Antifa is a loosely organized (at best) movement of primarily Anarchists and other assorted fringe types collecting under that banner. By their own ideology they abhor and reject authoritarianism.
They hold no seat of power nor even political sway, so are not in a position to appeal to authoritarianistic and ultranationalistic tendencies, nor strongly regiment society and the economy, nor forcibly suppress opposition, nor do they wish to. They do seek the forcible suppression of neo Fascists, of whom they are in opposition to.
These Fascistic types can be characterized as Trump and his base, since what they stand for and are willing to do fit the defintion you posted.
It is this that Antifa seek to take action to stop.