If I'm interested in content not on HN, I'll find it elsewhere. Why is it any different for YouTube? Now, if you'd said to me that Google doesn't prioritize other content, I'd argue that those wanting to talk about other content always have smaller forums to do it in, they just might be more local or viral in other ways. To me it's the difference between national broadcast TV "I'll know it's wrong when I see it," and the ability to post flyers or posters in a community. One is local and the other is wide-reaching.
There is, I suppose, another alternative. If we want to promote fringe ideas, we should make it easier to organize and elect local candidates, abolish "first-past-the-post" voting systems that encourage a two-party system, and so on.
But currently scientific reasoning is an accepted system of analysis for evidence-based decisions, and most politicians rely on it in addition to emotional intelligence to alter and guide their behaviour and decisions, even as they generally do whatever the person with the deepest pockets says to do, whether because that person sponsors scientific research or conducts surveys and campaigns to sway public opinion.
The issue I have is that fringe opinion, like this, would reject cause-and-effect reasoning and evidence to rely almost entirely on emotions instead. And because it's controversial, people click on it and watch it and wonder about it. When there's no actual evidence to it.
We used to and probably still do, have courses in grade/high school on media literacy, or not believing everything you see on TV. We routinely, as an HN community, encourage reading source materials and deciding for ourselves. But we're a small community, and for YouTube, it's easier to say "it's not our problem any longer" than it is to try and provide counterpoints to the material.
There's a serious risk to YouTube to not pull this down, because of how the feedback loop of their algorithms will encourage folks who like this to watch other fringe videos, until eventually they think all of YouTube, or the world, thinks the same way as these commentators. It's easier in the short term to pull down the content than to change how the algorithms work to make them somehow less relevant. And back to my original point, folks who want this content, they will find it, regardless of what platform it's on.