I don't know, but don't you think perhaps there's a reason other than being addicted to an app that makes said younger brother want to be hooked to his phone all day, avoiding things like school? Perhaps it's just an avenue to escape something.
As another example, many F2P mobile games have the concept of "whales", individual players who invest absolutely unwarranted amounts of money into a game. Is this healthy player behaviour? Of course not and no game dev would say it is. Is there a majority of players who manage to play in a healthy way? Usually there is. Nevertheless, a significant part of funding for those games comes from the few whales, so the game only works as a business because some players get addicted to it.
My friend's young 4-5-year-old, for example, the child of two model parents, has had similar behavior. Her screen time is very limited, but once there is a smartphone or a tablet in sight, she's inconsolable.
She does not use anything like TikTok, just watches some parent-curated videos.