The practical effect is that if you're accused of being a member of "antifa" there's no coherent way to demonstrate you aren't, and certainly people opposed to fascism, whether or not they themselves identify with the label, are going to get accused of being "antifa."
(Also, to be clear on the news, he simply said that he intends to do this in a tweet - even if it were a coherent thing to do, there is no procedure for designating domestic groups as terrorist organizations.)