and is saying that it somehow means Facebook owns it

owns what?

It does mean that Facebook owns the copyright to a software component that TikTok (apparently) also uses. They licensed it under a free and open source software license, so anybody (including TikTok, including you, including me) can download it and use it as part of their own software. It does not mean Facebook owns TikTok, nor that Facebook owns any other piece of software this is used in.

If you want to see more examples of similar notices: if you are using Firefox, enter about:license into your URL bar. If you are using Chrome, enter chrome://credits there. You’ll find that these browsers, like many pieces of software nowadays, rely heavily on open source components developed by third parties.