from the article it says "Today, 6 media giants control a whopping 90% of what we read, watch, or listen to.". there is a heavy handed content moderation policy on all major social media platforms. it is a rare day when I don't see or hear of blocked suspended or banned content on social media.
They are not just aggregators, but also publishers. They ban/censor certain content, they create algorithms to shape the exposure of information, they control monetization, etc. they have far more influence on “American media” than the six listed here.