That's a fair argument, and I can't dispute it. Thinking about this made me reflect a different way to put it. It shouldn't be one vs. the other, but rather, 4chan (and similar forums) are a meta reading of the news and train you to see blind spots or what's not being reported, listen to arguments from both sides of the aisle, and sprinkles a healthy dose of skepticism. After all, most of the discussion revolves around an extract from a maintream newspaper article or mainstream personality, so you're exposed to the "mainstream viewpoint" no matter what.

So the bottomline of what I'm trying to say would be: if you exclusively consume mainstream news, you're probably more vulnerable to adhere to a narrow viewpoint, and maybe not even know there are people who disagree.

Similar to how HN operates when discussing a blog post, especially clickbaity posts -- when reading the comments you may see that there's more to the story or that the author is wrong about something. Sometimes that has helped me get a more nuanced perspective on a specific topic.