In other words, do I think you should be able to say that "the earth is flat"? Yes. Do I think that social media should give more visibility to content that says "the earth is flat" than to content that says "well it is not flat" just because the first generates more engagement? No. It's not the speech that's harmful, is the selective amplification that social media does.
That's precisely the point I'm making, nobody needs to decide what is a lie or what is not. Social media amplifies lies not by knowing what is a lie, but because of their nature. Lies generate more controversy and more discussion, true facts generate agreement but not a lot of discussion and engagement. Reduce the importance of "engagement metrics" in the algorithms that show you content, make them as straightforward as possible (chronological order for example), reduce or eliminate suggestions of "content you might like" and only show content you explicitly subscribed to. At no point anyone needs to say "this is a lie/truth", you just need to change incentives.
The only way you can have a free exchange of ideas is if people are allowed to disagree, and yes this includes basic "facts" of reality.
You think speech should be censured, removed, and controlled? Okay, your speech first.