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.
No mass is 100% identical though, so "you can't use some individual actions to project on the group they chose to be part of and that chooses to accept them" really just makes the concept of groups useless.
"No, that specific action wasn't covered by our shared intent, so obviously we will accept responsibility for it" is something I believe pretty much everybody will agree on after the action has happened and has resulted in negative feedback. Had it produced applause and achieved the goal of the group, they would have celebrated it.
You certainly can't cherry pick them (one instance of violence versus billions of instances of peacefulness, for example), apart from "group that chooses to accept them" not applying here.
> Had it produced applause and achieved the goal of the group, they would have celebrated it.
Had there been anye instances of violence by people calling themselves antifa, those seeking to defend the ongoing, systematic violence would have invented instances of violence of people they call antifa. See how that works, or rather, how it doesn't?