My personal hypothesis (based on nothing) is that TikTok just uses the very strong signal of watch time. If you watch a clip all the way, or multiple times, that's good. If you skip early - that's bad.

When I had Netflix I remember being frustrated that Netflix would recommend me shows "based on" content I had watched for a few minutes, decided I didn't like, and backed out of. Why would you recommend me content if you have a strong signal I dislike it?