That last statement is meaningless given the crazy levels of security they have on keeping people in. If they took away all the restrictions on leaving then the numbers would go through the roof.
I don't think they are, they're more akin to forums.
In my mind, social media is where you follow people and people broadcast their lives. That's the social aspect of it.
With Reddit and Lemmy we follow communities on topics we're interested in.
I do get the arguments for it to be social media but that just makes the category way too broad, as you could argue any site with a comment section is social media.
How is this a controversial take? If you need a wall to keep people in or attempting to emigrate makes you a "defector", or you've built up a huge surveillance network where your neighbours or even partners can report you for bullshit "crimes" , you're an authoritarian state.