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.
So when you say 'From what I can tell antifa is anti-fascism' - what does that mean? America doesn't have any serious fascist groups - from what I can tell the ruling powers are decidedly corporatist and the first alternative philosophy seems to be light/moderate socialism. Second is maybe libertarianism.
So who/what do you think the anti-fascists are opposing, and what would they espouse if they ever decide there are no fascists for them to define themselves against?
Corporatism was inseparable from fascism in Italy and Germany. The exploitation of the profit motive is one of the primary reasons that so many people overlooked the atrocities.
Fascism is the reason so many companies like IBM, Hugo Boss, L'Oreal, Koch Industries, Audi, Porsche, Adidas, BMW, and countless other extant corporations have dark histories from supporting the German extermination camps to utilizing their slave labor to build their products.
Quoting a few key sentences from Wikipedia:
Fascists believe that liberal democracy is obsolete and regard the complete mobilization of society under a totalitarian one-party state as necessary to prepare a nation for armed conflict and to respond effectively to economic difficulties. Such a state is led by a strong leader—such as a dictator and a martial government composed of the members of the governing fascist party—to forge national unity and maintain a stable and orderly society. Fascism rejects assertions that violence is automatically negative in nature and views political violence, war and imperialism as means that can achieve national rejuvenation. Fascists advocate a mixed economy, with the principal goal of achieving autarky (national economic self-sufficiency) through protectionist and interventionist economic policies. [0]
America doesn't have a serious lobby that believes in those things. There isn't a lobby that is serious about autarchy, there isn't a lobby calling for complete mobilisation and there isn't a lobby calling for a one party state. Apart from maybe the anti-facists I don't know of a lobby promoting political violence. The war and imperialism stuff is possibly true, but that isn't a new thing in American politics - America has been at war my entire lifetime and mostly in the same set of middle eastern countries.
The only link between fascism and American politics is that Trump is popular in the Republican party and is happy to stand up and say that the globalism pendulum has swung too far. That is a tenuous link to fascist ideology.