I would argue that in this case the platform itself is expressing speech by ranking, recommending and promoting certain content. A foreign entity has no such first amendment right- we have had restrictions on foreign ownership of news media for decades now.
I think it’s an interesting issue especially now that you have TikTok users who think they’re being treated unfairly moving to a pure Chinese platform RedNote and encountering actual censorship. https://amp.cnn.com/cnn/2025/01/16/tech/tiktok-refugees-redn....
And now unconfirmed reports that RedNote is considering segregating the new American users from the Chinese users, ironically so Americans couldn’t influence Chinese users - https://arstechnica.com/tech-policy/2025/01/rednote-may-wall...