Isnt taking the app down today just for show? It seems like the ban wasnt going to be enforced? https://apnews.com/article/tiktok-ban-trump-executive-order-...
TikTok and/or Apple and/or Google probably took the app down themselves. They talk to their lawyers, and tell them "no, Trump/Biden said they weren't going to enforce the law" and their legal counsel goes "no, no, no, you can't go by that, obey the law or you are opening yourself up to huge liability" and that's it.
Specifically the statue of limitations is 5 years. Even if Biden and Trump both decided not to enforce the law, the next president could still go after app stores for hosting it, and impose all fines.