I've never understood this statement. Do you have any trusted sources of news? Are you not able to read them directly on the web?

Maybe you're just saying you want news about X, and you're not sure where to find a source that can tell you about that, and this is where you find "control". What kind of control are you talking about here? Deliberate? Accidental? Algorithmic? Censoring?

They are a filtering function applied on top of primary content producers.

However, in the case of these six primary content producers, the distributors don't really do any filtering.

So, yes, the premise of the OP is correct. Six companies produce ~all of the media you consume, with a couple of distributor companies that deliver that media to you. If you're concerned about homogeneity of thought in media, you should be looking at the market share of the primary producers.