By stating "Antifascism and anticapitalism are not terrorism" you create a hard statement: Anything labeled as anti-capitalism is not terrorism.
And far too often people downplay their violence and ruthlessness by using those labels as an excuse.
Yours is simply a logical fallacy. Antifascism and anticapitalism are not terrorism. That's a fact, they are ideologies expressed in a multitude of ways, most of which absolutely pacific. This doesn't mean that it's impossible for terrorist acts to be committed in the name of those ideologies, but also this doesn't make those ideologies inherently equivalent to terrorism.
islam is not terrorism. christianity is not terrorism.
are you arguing that those statements are false?
because terrorist acts have been done in the name of both.