They could have easily stated Ngo's affiliations at the time without compromising his identity. You're giving Antifa way too much credit. They aren't a well-oiled resistance machine meant to fight fascism. They are a loose collection of mostly young, bitter, violent idiots who have deluded themselves into believing they're fighting a war against a second Nazi regime, and that anyone on the right is their enemy. They feel that this is a revolution, and that violence is justified for that reason.
It's amazing to watch the delusion in this thread.
"Antifa has never attacked anyone without being provoked."
"Antifa only attacked Ngo because they knew he was a fake journalist aligned with violent alt-right groups, and not because they were bitter that he was on the right and writing mean articles about them."
"Antifa never killed anyone, but the KKK killed thousands, so obviously Antifa is nonviolent."
"Antifa is not interested in silencing free speech."
Antifa is a shit organization of violent lunatics with a misguided belief that they're freedom fighters.
Responsible people on the left can condemn Antifa and its actions without conceding a single political or social belief. It's okay to admit that certain forces on your side are dangerous and counter-productive.