The data isn't the most valuable part. Which videos someone likes to watch could be worth something, but given how much easily available information there is already online this isn't a particularly valuable (or sensitive) thing.

The much bigger issue is the algorithm that decides what content to show people. The CCP could decide to leverage that black box to push content that influences users in the ways they desire. That's why the CCP passed a new law to allow them to ban the sale of the algorithm (and they've expressed their willingness to use that to actually block the sale of the TikTok algorithm). Their goal is to either keep control of that so they can influence international users or to keep their potential involvement opaque so that anyone who engages with the algorithm continues to be unaware of the fact that the CCP can push content to any user they want.