I'm not a TikTok user, but I'm assuming the recommendation engine is there to keep eyeballs on more ads for longer. Maybe we should be regulating how often and how many ads can be shown on social media, especially to teens and kids.

They are arguing its not the recommender that is unique it is the network effect.

If it was only network effect, then how did TikTok grow in a space where Instagram and Youtube were already much bigger players? How did they gain that user base?

Network effect helps, but it only explains why they stay big, not how they got big

There really isn't that much making TikTok unique. Yes, their app is well designed. Yes, stitches and video replies make for great social/parasocial features because creators are actually interacting with each other and the community, almost like tumblr. But in my opinion those are reasons number three and two why TikTok is successful. Their recommendation algorithm is number one, by a wide margin.

Because back in a day, Instagram and YouTube were shitty products and by some metrics still are

No the primary answer is "teenagers do what other teenagers do". Remember we are advanced apes no more no less.

There is this curious word "influencer" which everyone uses but few ever think about what it really means.

That's not an answer, that's a thought–terminating cliché.