Lemmy has the ability to lock down forum creation, like on programming.dev which is the 8th largest lemmy site.
Social media has always been defined as being about people, not topics. People just don’t even try to use the right words though so you get ridiculous things like people calling something coincidental or unfortunate “ironic”.
By your definition every single news comment section is social media, which is clearly a ridiculous suggestion. Webchat, irc, literally anywhere there’s a comment section. That’s just clearly incorrect and so broad as to be a completely useless definition.