What you're missing is that while it's certainly true that there are plenty of small groups working from the antifacist position, the claim isn't simply that these groups exits- the claim (by the US gov) is that these groups are organized and funded in a top-down manner. Hence the "puppet master" style operations that they so hilariously failed at running against these groups.

The idea that they can find the folks organizing and funding these "cells" is a big part of the US gov's premise on how it is going to "fight" these "cells".

It's not stated as explicitly as that, so it's not surprising that folks don't pick up on that distinction between "distributed groups who agree on principle but don't have any material connections" and "top down, hierarchical structures funded by singlular sources".

But the folks running the US gov don't even see the folks who disagree with them as human, much less as having a legible and understandable grievance. So it's not surprising to me that they just project their own authoritarian and top-down organizational preferences onto other groups of humans doing stuff.

So sure, "antifa" exists. But it's categorically different than how the US government is trying to say it exists, and that's an important distinction.