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.