They're very different, and I understand what you're getting at comparing it to the hyper-manufactured perfectly glossy Instagram culture, but I wouldn't call TikTok 'authentic'.
Of course, Tiktok is large and there's many different subcultures there, but overall I think TikTok is heavily drenched in Irony. It's a stark difference to the very fake Instagram, but that doesn't make it authentic.
Are tiktok dances 'authentic'? They might have started as just innocent kids doing a fun little dance, but the moment anything turns into a trend I think it loses authenticity. The whole NPC live streaming trend[1] from a few years ago was anything but authentic. TikTok 'suffers' from the exact same paid 'influencers' promoting whatever garbage of the day, and even has its own version of affiliate marking spam with 'TikTok shop' junk.
[1]: https://theconversation.com/people-are-pretending-to-be-npcs...