Are you talking about pieces that were fully human-written with zero AI editing/rewriting etc? If so, what makes you think that false positives will happen there? They aren't looking for "writing styles" or emdashes etc. They are using watermarks and metadata.
If you're talking about people using AI to copy-edit text they manually wrote, this was explicitly called out in the article:
> A detected mark provides a signal that content was processed by Claude, but is not fully conclusive. Detecting a Claude mark tells you that the content may have been processed by Claude. It does not, on its own, confirm the full provenance of the content. For example: Claude may not be the original author. People often use Claude to proofread, translate, summarize, or convert files. The output can carry a Claude mark even if the underlying ideas, text, or data originated from another source; The content may have changed after Claude processed it. Marked content may be modified, excerpted, or combined with other material after Claude processed it.