The difference is that North Korea is a place, with an organization that claims to be its government. You can point to it on a map.

Antifa is an adjective that people with no connection to one another self-apply. I'm antifa, and I imagine you are too, but it doesn't mean that we've ever met or coordinated with one another in any meaningful way.

The word "antifa" is basically meaningless altogether, since virtually every person since the end of WW2 claims to oppose fascism.