Yes, the famous events that come from letting everyone speak: book burnings and death camps. If we could just burn enough books, we could keep books from being burned forever. If we silence enough people, we can keep people from being silenced.
Deplatforming doesn’t forcibly prevent speech, it just pushes it to the margins. You can still publish your book in favor of book burnings, but, in a deplatforming world, no major publisher would publish it, so you’d have to do a small independent print run that would likely be ignored.
This is the way the world has always worked. Publishers (including TV channels, newspapers, etc.) have always exercised discretion over what they thought was worthy to publish. What’s different today is that the human content moderation has largely disappeared, which has allowed the marginal voices (antivax on both left and right, QAnon, etc.) to flourish.
Edit: That’s not to say this content moderation has been perfect. It allows small communities to exercise behavior that the larger society finds abhorrent, such as refusing service to people of color. And so laws are passed regulating what small communities can do, which understandably pisses them off. Human systems are messy; there’s no absolutist answer that works, either in unbridled control or unbridled tolerance.
I'm not aware of any group intending to overthrow a government or otherwise do harm which was allowed to recruit and spread their propaganda as aggressively and freely as possible, and which was thwarted in their aims by nothing more than civil discourse. I don't know what experience he believes to be contrary to that classic argument, but that of the Nazis and Communists ain't it.
It certainly doesn't seem to work with modern propaganda or conspiracy theory. I'm certain QAnon and anti-vaxxers have been presented with arguments contrary to their claims, and those of white supremacists and anti-semites have been litigated for a century or more, and yet not only do they persist, but are experiencing a modern renaissance.
Show me an example where this proposition that "letting them all talk" is the best way to prevent organized violence and mayhem at scale. I can raise you book burnings and gulags and death camps galore.