On forums, people maybe be just talking about the thing that the forum is about. For example, if you're on a forum about Minecraft or cats, you're not going to be discussing differing political opinions — in fact, such conversations are usually frowned upon. This is different from your real life community, where you're going to be talking about all sorts of different topics.
A lot of real-life communities restrict discussions about politics too, and for similar reasons to online communities (not wanting to be broken apart by angry discussion). What I got from the video was that positive interaction with somebody you disagree with — even if, or maybe especially if, that interaction isn't actually about the source of the disagreement — is more important than actually facing the disagreement head-on, which usually just serves to make everybody angrier and more convinced of their own points.
My thoughts: I'm in a lot of small communities online (mostly forums and discord servers), and the ones I enjoy the most are the ones that are generally populated by people who have similar worldviews and cultural values, and that prohibit discussion of extreme points from the "other side" — not because I don't want to challenge my worldview, but because I think those communities challenge my worldview more. Spending time in them makes me feel positive about other human beings. I respect the people in them who have different opinions from me, and frequently have conversations with them that change my own opinion a bit.
On the other hand, going into the default feed on a larger social media site is like being hit in the face and then told that I'm supposed to be considering whether maybe I deserved to be hit in the face, really, which isn't something I usually feel particularly disposed to do. Sites like mastodon have enough controls to be able to make your feed into something that at least isn't actively radicalizing, but it means you can't follow anybody who disagrees with you, because those posts will be mixed in with anything from them that you actually do want to see. Your feed isn't like a forum with its own set of social norms about what kind of content is acceptable to post in what context. People you follow can't read the room before they post because the room isn't visible to them in the first place. So I do think that forums are much more conducive to being deradicalizing than a lot of other forms of social media, just because the nature of having distinct communities with separated pools of posts makes it a lot easier to interact with people you disagree with in situations where you agree with and respect them. But it also relies on the forum having the right rules and moderation — just being a forum isn't enough for it to be a healthy and respectful community.