You are technically correct.

The right term in this case is an oligopoly.

Semantically it makes little difference though.

Which generally requires collusion. There was no collusion. These are independent actors in a competitive space that arrived at the same conclusion independently.

People want free speech. You get free speech so long as your speech doesn’t unduly impinge on other’s liberties. That is the trade off we accept when we exchange the rule of force for the rule of law in a self governing, democratic society.

People are not complaining about business practices. People are complaining about not being able to say whatever they want over whatever medium they want even if other people get dead.

Mark zuckerberg, during one of the senate hearings, admitted that facebook coordinates with other big tech companies in certain situations.

In this situation?

I suppose that the article is talking about monopolies in their respective market (namely iOS app stores, Android app store, and "the cloud").

Unless you're quite tech savvy, you can't sign up for Parler anymore by using a competitor. So, while I mostly disagree with Greenwald, I don't think this characterisation is unfair.