Beyond the Trump point, I think there is a very real concern being raised by Greenwald that the supposed fourth estate has suddenly uncritically been parroting the words of "their team", verbatim. While this may have become more apparent recently given all the political change and even COVID coverage, there is a history of this happening, most crucially in the run up to the Iraq War before everyone turned against it.
But if you take Iraq as an example, the corporate media has always been a consensus building machine. The danger then when it is presented as a monolith arbiter of the truth, is that if you accept the news as default "true", then any bad actor can start pushing the population in an unhealthy direction for society.
The opposite also has issues: If you accept the media as default "false", then what to believe? What is actually true outside of your own lived experience?
I see Greenwald's concern less about Trump or the politics of the day, and more about the sad lamentation of the death of his profession. The destruction of belief in institutions is increasing at a quicker pace, and that doesn't leave us as a society in a good place. And lecturing/hectoring people about "don't believe fake news, THOSE OTHER GUYS are wrong" doesn't work, because you often turn the page, and encounter something you know to be the opposite of what is being presented from that same media source. Gell-Mann Amnesia again and all that.