A quick look shows me antifa is a "political movement". Like Nazi. Or communist. Saying antifa beat someone up or tried to burn down a police station is like saying communists beat someone up or Nazis.

No - it's like saying communism beat someone up. Or Nazism. Communism is a political movement, while communists are people who follow that movement.

Al, why do you get wrapped around such a stupid axle?

Because the people who criticize Biden for saying antifa is "an idea" typically forget why he said that. In the last election, Trump was going around saying that the FBI should declare Antifa terrorist organization. Which is profoundly dumb. It's like trying to declare communism as a terrorist organization. It's an idea or a movement, not an actual organization.

There are individual organizations that advocate the ideas of Antifa, of course - and if any one (or more) of them was found to be a terrorist organization, you could declare that organization a terrorist organization. But you can't declare an idea or a movement to be a terrorist organization....because it's not an organization, so the laws don't apply to it.

Which is what Biden (and a lot of other Democrats) were pointing out. You can declare Antifa to be a bad idea - and you can describe a "fight" against Antifa as a rhetorical device, much as we have a War on Drugs or a War on Poverty. But you can't have a criminal prosecution against "Antifa," any more than you can indict "Drugs" - those laws apply to individuals or organizations, not concepts or movements or ideas. Which is why the distinction matters, and why the government response is the way it is.