I agree with you, it could have a place in a toolkit of things to acquire a language with. But I don't think that's what they're marketing themselves as. Their tag line is "The world's best way to learn a language", which, personally, I wouldn't blame a person for reading and thinking "cool, I guess I'll just do this and finally learn a second language!". They didn't say "build your foundation in a foreign language" or "first steps in your language learning journey". They said "best way to learn a language", which, I'd say, is false and misleading.

I've never used TikTok, but I actually wonder if this hypothetical teenager would learn more following a ton of users in their target language or playing games on Duolingo. I'd be interested in that study.