Comment by huhtenberg

For anyone wondering who the text watermarking works:

  Instead of using an arbitrary random number generator
  to pick the next word, watermarking uses a secret key
  and a few words that come before to settle what
  word the model should pick.

https://www.anthropic.com/news/claude-text-watermark

* Edited the above from initially asking if anyone has any ideas how it works. Found the answer, rewrote the comment.