I don’t really see how that undermines anything, I just explained the abbreviation and mentioned how I mostly see it used. The only group I see screeching about how the “mainstream media” is biased against them is the far right.

Most people agree on which news orgs are “mainstream media”: https://www.pewresearch.org/short-reads/2021/05/07/broad-agreement-in-u-s-even-among-partisans-on-which-news-outlets-are-part-of-the-mainstream-media/

And an article on the topic of alleged bias: https://www.npr.org/2022/08/07/1115949410/republicans-have-long-feuded-with-the-mainstream-media-now-many-are-shutting-the