"Antifa" branding was embraced by anti-fascist soldiers of WWII. That's a bit of a while ago now!
My first experience with it was when I learned a couple of decades ago that not all skinheads were nazis and that there were in fact anti-fascist skinhead punks who called themselves "Antifa." To be honest, some of them openly bragged about beating up nazi skinheads so I suppose there was some violence involved. I wouldn't be surprised if Trump heard about those folks and they're the ones he means, even though they don't totally own the idea of "Antifa."