YouTube has editorial control over which videos are allowed to stay up, which videos are eligible to get monetized, and which videos are actively promoted. It's not just a web server with an index and a movie player.
And let's not forget iHeartMedia which owns around 850 US radio stations. Their stations reach nearly 100 million listeners a week, and nearly 250 million every month.
Without picking a side or making an argument, it's basically a repeatable factoid that gets people wondering how much collaboration, and potential illicit collusion, is really being applied to the stream of messages they are exposed to on a daily basis.
Personally, I think it's naive to blame some exact set of companies... but in reality I never would've thought about it until someone pointed out how severe the consolidating was, and in plain sight.
None of whom are major producers of their own content. They're just floating user-generated content and content from the corporations mentioned in the OP.
The six they list doesn't include YouTube, Facebook, Instagram, TikTok etc where a huge amount of media is consumed.
And would they count YouTube as "one" outlet? I would argue I have greater choice of amongst YouTube creators than I ever did compared to, say ABC, in 1983.
The analysis also seems flawed in that they evaluate movies based on box office sales. Which excludes giants like Netflix?
There may be issues here, but this article wasn't very good.