I guess if it's only specific individuals/groups that can't print books, it's restricting free speech, if nobody can, it's not.

Not really the same, because the article is not calling for banning any source of social media. How would you even classify social media? We are taking about ad infested hellholes with no incentives other than maximizing revenue, regardless of the content pushed.

The proper analogy would be banning books with certain content, which we already do. You can't distribute a book calling for a specific person to be killed or doxxing them. Doing this on social media in Ethiopia is encouraged, as it drives engagement and has lead to actual deaths of people I know. They have a policy not to moderate this content despite having the resources. Just like they have a policy to make the apps as addictive as possible.

More importantly, Facebook is not a "printed book", it is the printing press. It owns the internet. It's not remotely comparable. And that's why it is a threat to free speech

How is access to a wholly privately owned walled garden in any way relate to printing books? Private networks are by definition not public domain and thus are totally irrelevant to any discussion of free speech.

I would agree if it weren't for the complete transition to privately owned communication platforms. The answer to your question is actually quite simple: because communication via privately owned walled gardens is humanity's primary means of mass communication, just as it used to be printed media.

It would be as if printing presses were so complicated and expensive that the barrier to entry was so high as to price out everyone but a few select publishers. I wouldn't try to over-extend that metaphor though.

That folks have opted to interact (or been manipulated into it if we're really honest) with modes of communication that are outside of 1st amendment protections doesn't change either the spirit or the letter of the law. That would be like saying that if folks suddenly decided to communicate over transcontinental distances via morse code utilizing geophones and large explosions as the transmission mechanism so now the first amendment demands semtex should be broadly accessible to the public.