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.