An organization does have to have a clear set of shared plans, toward which it takes a program of coordinated action. That's what an organization is. It's about being organized. A bunch of random individuals who share some opinions, occasionally use similar tactics, may cooperate sporadically, and describe themselves using some term or gravitate toward certain symbols, is not an organization. And it's not clear that "antifa" even does most of those things.

Nor are a bunch of small groups a single "organization" if they don't have some kind of ongoing, systemematic structure for coordinating their actions.

I'm an atheist. That doesn't make atheism an organization.

What do you think _soviet_ are

https://en.wikipedia.org/wiki/Soviet_(council)

The communists were very centralized. They held world governments. Even the American communists were centralized.

There was a national Communist Party as well as several Cominterns. Communists are pretty big on democratic centralism.