There is an interesting meta discussion here but the parent is over-simplifying things.
> How we can allow a Chinese social media app in the west, while any non-Chinese social media apps aren't allowed there?
Easy. The laws are different.
"Non-Chinese social media app"s are not banned in China, just that if you run one it need to be licensed (https://beian.miit.gov.cn/) first before you can start servicing. Licensing is difficult since there's requirements about keeping data domestic, having physical presence should legal enforcement be necessary (i.e. there are people to arrest if something goes wrong), and complying with takedown requests (both copyright and political). Western big tech companies (rightfully) do not want to comply, so they do not get licenses, and thus have no presence. Attempting to "just provide service" without a license will result in blacklisting via the GFW as enforcement.
"Allow a Chinese social media app in the west" -- this is also more complex. If TikTok or friends violate laws in the west they are also liable for any punishment. For example, TikTok and WeChat comply with the GDPR in Europe and keep EU data local to the EU. If they didn't they'd be looking at a potentially huge fine and possibly getting banned. Similarly they also comply with copyright stuff like DMCAs. If they didn't, the FBI can seize their domain and compel ISPs to not resolve it just like the GFW (this has precedent and has been done before).
So the meta question becomes: Are the current protections in the west sufficient? To which the answer is probably no.
But in any case, in the free world, whether a Chinese social media app's presence is allowed to be maintained should not be dictated by ideology, but rather through real demonstrated evidence of misbehavior and/or harm (which is why research like this is important).