Antifa is a decentralized organization of people fighting against fascism. They don’t have a positive ideology, literally the only thing that unifies them is anti fascism. Communists, anarchists, and liberals may all call themselves antifa
Using that logic Republicans could call themselves antifa since almost everyone, Republicans included, is against fascism. It's when you define "fascism" as anything right of Trotsky that people start have issues with your ideology.