They (not Trump of course) had to walk it back when it turned out not to be true.
Is there some outside groups posing as others, possibly, but to blame a majority of problems on them is just BS.
I see a lot of mischaracterization of what is a category, not a group. From what I can tell antifa is anti-fascism, and somewhat characterized by people willing to take direct action.
This is the definition of the word, no matter how much lefists try to whitewash the term and shift the overton window.
I know a number of people in various countries who participate in Antifa action and who, while definitely left of center, are by no means anarchists or communists, and not even close to Marxists-Leninists or whatever other hard-left groups there are.