Neat. But it's not pre social media. Social media has been around since the 1980s.
That's why I alternately call it the "platform web" in which social media stopped running on open and interoperable protocols and started being walled gardens on commercial platforms.
And to your other point - it shouldn't be solely up to forum admins to opt-in to the Fediverse, end users should be able to subscribe to the forums they belong to with Fedi-powered tools.
The Fediverse could and should be the place where all those things can converge but I'm pointing out that it currently is failing to be that.