I see people saying this a lot, but I've also seen videos demonstrating that you can easily post and search for Tiananmen Square content. I don't use Tiktok myself but it seems like this is basically untrue.
But this isn't new either, western services operating abroad will often comply with local laws, which includes country or region specific laws on acceptable content. Google pulled out of China for a good while because they didn't want to, but they eventually cracked and complied with their content laws. Of course, by then the competition was dominant already.