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.
Wikipedia points to many resources about Antifa movements, starting from here:
https://en.wikipedia.org/wiki/Antifa
And you only care to pick one narrow definition.
You got it backwards.
The only definition from that page that is not related to some extremist marxist/anarchist movement is this article:
https://en.wikipedia.org/wiki/Anti-fascism
And even that one uses the logo of the Antifaschistische Aktion. None of the other historic Antifascist movements described in that article are in any way relevant today.
Today, "Antifa" is synonymous with "Antifaschistische Aktion".